It is this quality of Fortunato's that Montresor can use to manipulate him, and his pride, itself, will actually prevent Fortunato from realizing that he is being manipulated by Montresor. 

One of the ways that Montresor manipulates Fortunato by using the man's pride against him is by being somewhat self-deprecating and then admitting that Fortunato is more qualified than he on the subject of wine.  Concerning the supposed pipe of Amontillado, Montresor says, "'I was silly enough to pay the full Amontillado price without consulting you in the matter [...]."  He implies that he behaved hastily, without due reflection and consultation, and that he would have been smarter to have gotten Fortunato's opinion before he purchased the wine.  Montresor knows that Fortunato will not be able to resist the opportunity to gloat over Montresor's mistake.

Likewise, as the two men descend into the catacombs, Montresor repeatedly insists that they should turn back for the good of Fortunato's health.  The walls are crusted with nitre (potassium nitrate) which makes it difficult for Fortunato, who is already somewhat ill, to breathe.  But Fortunato seems not to want Montresor to be right about anything, and so he denies, over and over, that he is suffering.  Moreover, Montresor tells Fortunato, "You are rich, respected, admired, beloved; you are happy, as once I was.  You are a man to be missed.  For me it is no matter."  Again, Monstresor speaks slightingly of himself in order to flatter Fortunato's pride, and so Fortunato, proudly, rebuffs Montresor's expressions of concern.

Montresor is the protagonist and narrator of "The Cask of Amontillado" by Edgar Allan Poe, and he is clearly not sane. Despite that fact, he is quite clever. He feels he has been somehow injured by a fellow wine connoisseur named Fortunato, and Montresor determines that he has been pushed too far and must now take his revenge on his colleague Fortunato.

Two things are crucial to Montresor's plan working, and both of them involve his use of psychology to make things happen. The first is that no one at all must be in his house so he can dispose of Fortunato as he wishes without fear of discovery. It is Carnival time, and of course Montresor is perfectly bright enough to know that all of his servants would rather be out celebrating than doing their jobs on his estate. Montresor makes a brilliant move:

There were no attendants at home; they had absconded to make merry in honour of the time. I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.

This is a bold example of reverse psychology. Montresor does not say he even checked to see if they left; he simply knew they would all be gone. This is a great example of Montresor using psychological trickery, but it is also an indication that in some respects Montresor is capable of rational, even exceptional thinking. 

The second thing that must happen if Montresor's plan is going to succeed is that he must find a way to lure the unsuspecting Fortunato not only back to his estate but into the crypts below the house. To do this, Montresor appeals to Fortunato's weakness--his pride.

He had a weak point --this Fortunato --although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; --I was skilful in the Italian vintages myself, and bought largely whenever I could.

Because Fortunato is so certain he has the best wine palate in the city, Montresor simply lies and tells Fortunato that he has a cask of Amontillado, something Fortunato could not believe without tasting it. Every single time Montresor senses that Fortunato is wavering in his resolve to follow Montresor to the non-existent cask of wine, Montresor invokes the name of Luchresi, a man Fortunato claims "cannot distinguish Sherry from Amontillado."

This appeal to Fortunato's excessive pride does work (and is something that might have worked on Montresor, as well, as he is equally proud of his wine palate) and he follows Montresor all the way to his doom. One added strategy is that Montresor consistently gives wine as "medicine" to Fortunato for his cough, getting Fortunato drunk enough that he does not question Montresor's acts as much as he might.

This deliberate and rather intricate plan relies on Montresor's keen mind and knowledge of human nature, something quite confounding in a man who is obviously not sane.

Montresor takes a long time to fashion an elaborate lie that will entice Fortunato into the catacombs beneath his palazzo, where he can murder him. In the first paragraph, Montresor says

At length I would be avenged; this was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk.

He wants his plan to be foolproof. When he encounters Fortunato celebrating on the streets during the carnival season, Montresor tells him his finely honed falsehood.

I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”

“How?” said he. “Amontillado, A pipe? Impossible! And in the middle of the carnival!”

“I have my doubts,” I replied; “and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain.....As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”

A pipe contains 126 gallons. Neither of these men would want the equivalent of 500 quart bottles of Amontillado sherry for personal consumption. They are not even sherry drinkers. The word "bargain" is what captures Fortunato's interest. He knows Montresor only bought the wine for resale. Many first-time readers of the story assume Fortunato wants to taste the delicious wine and show off his connoisseurship. This is not at all true. He is interested in the "bargain." He is a rich man. He could buy up the whole cargo of big oak barrels of fine Spanish sherry and make a small fortune. The wine only improves with age, so he could take his time about bottling and selling it by the case, probably to those British and Austrian millionaires Montresor mentions in his introduction. 

Fortunato doesn't need to taste Montresor's wine at all. He could go to the harbor and find a newly arrived Spanish ship with ease. There would be a whole shipload of Amontillado to sample, and he could make the deal on board. But Montresor has foreseen that possibility. He inserts the name Luchesi so that, if Fortunato declined to come to his palazzo immediately, Montresor could continue on his way on the pretext of consulting another connoisseur, and possible buyer, about his Amontillado. Fortunato can't let that happen. He must accompany Montresor to his home to keep him from talking to Luchesi. Otherwise, Fortunato would be competing with Luchesi in buying up the imaginary cargo of imaginary wine.

It was essential for Montresor to get Fortunato to his palazzo right away. Otherwise, if there was any delay at all, Fortunato could find out there was no Spanish ship, no Amontillado. Montresor would not only lose the chance to kill Fortunato, but he would arouse his suspicions. That would make it infinitely harder to entrap him at some time in the future. Montresor does not actually say that he has bought a pipe of Amontillado from a Spanish ship. He says:

"But I have received a pipe of what passes for Amontillado, and I have my doubts.”

The cunning Montresor has left himself a loophole. If necessary, he could make up some barely plausible lie, such as that he bought the pipe from a Venetian who had had it in his cellar for some time. But he doesn't have to do that. Fortunato imagines a Spanish ship full of big barrels of gourmet sherry which he can buy at a bargain price and sell at a great profit. He does not question Montresor any further because he does not want his gullible, trusting friend to suspect that he would be interested in buying any of the wine himself. He can go to Montresor's palazzo, taste the wine, make sure it is genuine, then get away and find this Spanish ship. No doubt he already plans to tell Montresor the wine is only ordinary sherry, just to eliminate him as a buyer. Montresor could only be so anxious to get an expert to sample his wine that night  if he intended to buy more while it was still being offered as a bargain. If it really is just ordinary sherry, Fortunato can forget about it. If it is genuine, he can buy up the whole cargo.

Montresor knows Fortunato thoroughly. He has had plenty of experience with this man. He knows what his shrewd, unscrupulous friendly enemy is thinking and planning. Fortunato has swallowed the whole cunning lie hook, line and sinker. He wants to rush to Montresor's palazzo as quickly as they can get there.

“Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.”

Thus speaking, Fortunato possessed himself of my arm; and putting on a mask of black silk and drawing a roquelaire closely about my person, I suffered him to hurry me to my palazzo.

Yes, Montresor achieves exactly the kind of revenge he wants. He explains what he wants in the opening paragraph of the story, and by the end of the story he appears to be fully satisfied with what he has done. In the opening paragraph he states:

At length I would be avenged; this was a point definitely settled--but the very definiteness with which it was resolved precluded the idea of risk. I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

In more simple language, he wants to kill Fortunato without getting caught. The word "impunity" suggests that he wants to feel completely safe not only from exposure but even from the slightest suspicion of being guilty of the crime. He also wants his victim to be sure that it is he, Montresor, who is responsible for his victim's murder. In other words, he doesn't want to do something like sending Fortunato a bottle of poisoned wine or hiring some assassin to kill Fortunato in a dark alley. Montresor wants to do the deed himself with impunity and have Fortunato--but only Fortunato--aware that he is killing him for revenge.

Montresor has a lot of problems achieving his perfect revenge, and his coping with all his problems is the essence of the story. He lures Fortunato down into the catacombs and chains him to the granite wall. He manages to do this without being recognized by anyone in the streets above. Fortunato was drunk when Montresor encountered him, and Montresor keeps him drunk until he has him in chains. Then Fortunato realizes he is in deep trouble and quickly sobers up. This was necessary for Montresor to be sure that Fortunato knows what is happening, why it is happening, and who is doing it. 

It is noteworthy that Fortunato never calls Montresor by name until he is chained to the granite wall. Then he cries:

"For the love of God, Montresor!”

“Yes,” I said, “for the love of God!”

Montresor says "Yes," because he now has proof that Fortunato recognizes him as his killer. Montresor adds, "...for the love of God!" because he is pleased to have his arrogant victim begging for mercy.

At the very end of the tale Montresor states:

Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. In pace requiescat!

The fact that no one has discovered the bones in fifty years proves that no one has ever discovered the wall or ever suspected what is behind it. Montresor has achieved his revenge with "impunity." The purpose of getting revenge was to rid himself of all the painful thoughts and feelings that made him want the revenge in the first place. Now that he is fully satisfied, he means it sincerely when he says, "In pace requiescat!" (Rest in peace.) The Latin words are meant to convey the idea to the reader that Montresor did achieve the kind of revenge he wanted and feels utterly cleansed of his hatred and rage.

In Poe's classic short story "The Cask of Amontillado," Montresor does not go into specific detail regarding the exact reason he decides to murder Fortunato by entombing him alive inside his family's vast catacombs. In the first paragraph of the story, Montresor mentions that Fortunato had caused him a "thousand injuries" and "ventured upon insult," which motivated him to take revenge. Although Montresor is being vague and ambiguous, the reader can infer that he is a proud man who is extremely sensitive and vengeful. As the story progresses, Montresor approaches Fortunato, who casually insults Luchesi without hesitation. Fortunato's arrogant demeanor and flippant insult suggest that he may have done the same to Montresor. If Fortunato is willing to slander Luchesi behind his back, it is likely that he did the same to Montresor.

There is also evidence to suggest that Fortunato may have undermined Montresor's business ventures. The reader is aware that Fortunato is a rich man, and a pipe is a rather large quantity of wine. When Montresor informs Fortunato that he has purchased a pipe of the extremely rare Amontillado, Fortunato jumps at his chance to authenticate the wine in hopes of purchasing more. Fortunato recognizes that Amontillado is extremely rare during the carnival season and hopes to take advantage of the situation. Fortunato can certainly afford the great quantity of wine and make a profit selling the Amontillado.

If Fortunato plans on authenticating the wine to purchase more of the Amontillado before the merchant ship departs, the theory that he previously undermined Montresor's business practices holds weight. Additionally, Montresor is no longer the wealthy man he once was and may blame Fortunato for his financial struggles. However, we can never be certain of Montresor's exact motivation based on such circumstantial evidence.

Montresor is very serious about the subject of revenge and views it as a moral obligation to right the wrongs suffered by one's enemy. Montresor's ideology concerning revenge corresponds to the motto written on his family's coat of arms, which reads, "Nemo me impune lacessit," or, "No one provokes me with impunity." Montresor also has a precise definition of revenge, which he describes in the first paragraph of the short story. Montresor mentions that, when exacting revenge, it is imperative that one "must not only punish but punish with impunity." He is essentially saying that it is necessary not only to punish one's enemy but also to do so without being caught or suffering the consequences of exacting revenge. Montresor goes on to say that an avenger will not feel satisfied unless the victim knows who is responsible for their punishment. After describing the ideal conditions involved in avenging one’s enemy, Montresor elaborates on how he committed the perfect crime by getting revenge on Fortunato. Montresor's calculating scheme and horrific crime emphasize the seriousness of his attitude toward revenge. Montresor's actions correspond to his definition perfectly, and Fortunato's fate is disturbing and shocking.

"The Cask of Amontillado" is all about Montresor's revenge: the terrible revenge he wreaks upon the hapless, misnamed Fortunato. We're never told exactly what Fortunato has done to Montresor that warranted being walled up alive inside the catacombs. But whatever it was, this unspecified insult has driven Montresor into a frenzy that can only be satisfied by some serious payback—the most terrible revenge you could possibly imagine.

In his warped imagination, Montresor feels that revenge is not just necessary, it is the right thing to do. Revenge is like a moral imperative for Montresor, something he simply has to do if he is to redress the "thousand injuries" that Fortunato has heaped upon him over the years. But if Monstresor's going to gain revenge, he needs to do it properly. That means making sure that he gets away with his crime. His elaborate revenge plot has clearly been carefully planned and worked out to the last detail. This is to ensure no one will ever suspect Montresor of having carried out this wicked, dastardly deed.

Poe doesn't specify exactly why Montresor wants revenge. All that we are told is that Fortunato insulted Montresor in some unspecified manner and has subjected his nemesis to a thousand injuries. In examining Montresor's motives we need to bear in mind that we only hear his side of the story. There could be all sorts of reasons why he would choose to exact such a terrible revenge: some trivial, some more serious. At the very least, there's no doubt that Montresor feels deeply hurt and offended by Fortunato and is itching to have his revenge. That he should choose to exact this revenge in such a grotesquely horrific fashion indicates that, for Montresor at least, this is a very serious matter indeed.

Montresor believes that a person should take revenge in a calculated way.

Montresor states:

A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong. (enotes e-text p. 4)

In order to get “redress” (translation: payback), one needs to take time and make sure that the person who is in the wrong knows that he is taking revenge.  Montresor plans to punish Fortunado "with impunity" (in other words, without risk of punishment).  Montressor identifies Fortunado’s weak point (wine) and uses that to get him.

Monresor claims that he waited patiently for revenge.  Montesor believes that revenge best when the victim does not suspect it.  He is willing to wait, to take his time, to make sure his victim is off guard.  As he points out, he does not make threats.  He waits.

You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk. (enotes e-text p. 4)

Basically, he is saying that he took the time to make a good plan, and therefore there was no risk in carrying it out.

Montresor, Poe's unreliable and hyperbolic narrator, claims that he seeks revenge after Fortunato has added insult to injury.

In the exposition of Poe's Gothic tale, Montresor claims that he has endured "the thousand injuries" that Fortunato has committed against him; however, when his enemy has "ventured upon insult," he states that he can bear no more, and must be avenged. Having decided upon revenge, Montresor commences his intricate plan to approach Fortunato during the Carnival season when Fortunato's disappearance should not soon be noticed. Also, Fortunato, who should be at least somewhat inebriated from celebrating, will be more susceptible to Montresor's luring him into the catacombs on the pretext of tasting the Amontillado. 

Montresor's plan is effective as he succeeds in tempting his enemy Fortunato into the damp "vaults." Further, Montresor exploits Fortunato's desire to outdo his rival Luchesi by tasting the Amontillado. Montresor also feigns concern for Fortunato's health because of the dampness of the cavern walls and repeatedly suggests that they turn back. But Fortunato, who will not be outdone later by Luchesi or anyone else, insists that they keep going forward. As Montresor knows, Fortunato is a rapacious man who wishes to taste the Amontillado and judge it before his foe Luchesi has any chance to do so.   

Montresor decides to seek revenge against Fortuanato because he believes that Fortunato has insulted him. The story says "the thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge."  We are not told the specifics of this insult.  The story leaves the reader to wonder what the insult was and if it ever actually occurred.  The story also describes Montressor's family coat of arms and moto.  The coat of arms depicts a large foot crushing a snake that has bitten the heal of the foot.  His family motto states "no one attacks me with impunity."  This tells the reader something of Montressor's character.  He feels that he must punish any offense.  Montressor does not like Fortunato and feels he has put up with him long enough.  Finally, Fortuanto insults Montressor in some fashion and Montressor's anger boils over.  Once again, we do not know if this offense ever really occurred.

The only thing Montresor tells us is that Fortunato insulted him, but we never learn what Fortunato supposedly did. Since Montresor is the narrator, he isn't very reliable, so it's possible that Fortunato did nothing to Montresor. The offense could be in Montresor's mind only. His family motto is no one does anything to a Montresor without the offended Montresor taking revenge. So it's a family tradition in the Montresor family to avenge any and all perceived offenses.

Poe intended to write a horror story about a man who commits a totally fiendish murder. The murderer, Montresor, had to have an exceptionally strong motive for doing what he did. Montresor claims to have been injured a thousand times. Poe is able to avoid having to give examples of what these injuries were. The tale is presented as a translation of an old manuscript detailing a crime committed at least fifty years earlier. Poe therefore poses as only the owner and translator of this old manuscript; he doesn't have to know anything about the thousand injuries--but the fact that Montresor was injured so many times (assuming we believe him) not only suggests why Montresor plans and executes such a terrible act revenge but also suggests that this Fortunato must have been a terrible man who deserved what he got. Poe's problems in writing the tale included keeping the reader somewhat sympathetic for a man who was capable of burying another man alive and leaving him to die of starvation. Poe deals with the important question of motivation in his opening sentence, because motivation is the most important consideration in any story, and Montresor's motivation for committing such a heinous murder needs to be made understandable and plausible.

THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.

This says everything that is necessary in a single sentence. Montresor's conduct from here to the end of the story will be driven by two forces, (1) the thousand injuries he has received, and (2) the fact that this proud man has made a vow to obtain revenge. The vow might be considered a more important aspect of his motivation than the thousand injuries, especially since Montresor says he had already borne them as best he could. Yet it must have been the "thousand injuries" rather than the "insult" that made Montresor vow revenge. The insult might have been trivial and might have only triggered the pent-up hatred caused by the thousand injuries. 

Revenge for wrongs done to him in the past is the motive--unexplained as it is, at some point in Fortunato's life, he has wronged Montressor; for that, he wants revenge and the plot in the tombs was the best way for him to have it. That is about as brief as you can make it. Does that work? Brenda

We never know what exactly Fortunado has done to so enrage Montressor.  He does not ever explicity say what the man has done; the only motivation appears to be an abhorrence to the wine-snobbery and borishness of his nemesis.  All we know is that in his "confession" (to whom is a matter of contention) reveals only his hatred and loathing of his victim. 

Montresor has "vowed revenge" for what he calls the "thousand injuries" of Fortunato.

What these injuries are the reader never learns. Instead, Poe's unreliable narrator describes how revenge must be planned so that there can be no element of risk. According to Montresor, revenge is only complete when it is done "with impunity"; that is, there are no consequences felt by the avenger for his act. Yet, while the avenger must remain unknown to the authorities or any one besides the victim, the victim must be made aware of the avenger in order for the act to be truly revenge.

Poe's narrator enacts his revenge precisely according to his blueprint, luring the vain and arrogant Fortunato into the Montresor vaults where, supposedly, a large cask of Amontillado, a variety of sherry, is stored. Because it is the Carnival season, it is unlikely that any of the revelers will pay attention to Fortunato's departure; moreover, people are all in costume, so recognition of perpetrator and victim is difficult, if not impossible. In addition, Montresor's servants are gone, so they cannot know what their master has planned.

As they enter the vaults, Montresor picks up two lighted torches and gives one to Fortunato, pointing out to him the dampness of the walls and the niter upon them. When Fortunato coughs in this damp atmosphere, Montresor feigns concern, revealing a vague sense of his resentment while playing to his foe's ego: 

"Come...we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi--"

With dramatic irony Fortunato replies, 

"Enough! The cough is a mere nothing; it will not kill me. I shall not die of a cough."

Finally, after a series of twists and turns through the niter-covered apertures, Montresor lures Fortunato into a small recess by furthering his intoxication with Medoc. Then, Montresor fetters his victim to the granite in which two iron rings hang. With stone and mortar, he walls up the entrance to this niche.
His revenge completed, Montresor boasts to his audience that for fifty years no one has discovered his crime. Therefore, he has achieved perfect revenge.

Unfortunately, there is not really a satisfying answer to the question of motive in "The Cask of Amontillado" by Edgar Allan Poe. There is an answer, it is simply not a good enough answer to justify what Montresor does to Fortunato. Montresor tries to explain his thinking in the opening lines of the story:

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitively settled -- but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

What he wants us to believe is that Fortunato had somehow done him thousands of "injuries" which Montresor has managed to graciously overlook; then Fortunato insulted him and that was all Montresor could take. The insult must, of course, have been monumental to overshadow (be worse than) a thousand injuries, so when we read the rest of the story we are looking for signs that Fortunato really does bear some kind of venomous hatred toward Montresor.

Unfortunately for Montresor, we find nothing of the sort. Instead we watch Fortunato greet Montresor like an old friend, willingly follow Montressor to his house, and even joke with him in Montresor's underground crypt. It is true that Fortunato is full of pride and therefore falls into Montresor's cunning and well conceived trap; however, we find no evidence that Fortunato has any negative feelings against anyone but his rival, Luchesi. Montresor's claim to motive, that he could no longer bear the consistent insults by Fortunato, does not seem to be substantiated by the facts. 

It is our nature to expect a heinous crime to be precipitated by a heinous grievance because then at least it makes sense. Without that, such a foul act seems monstrous and unprovoked, and it is the same kind of reaction we have to school shootings and other senseless acts of violence. Since we do not have a motive sufficient to explain the crime, we can only assume that Montresor is not sane, for a same person would never commit such an atrocious act.

Montresor says that his motive for killing Fortunato was that Fortunato had insulted him.

We actually do not know what Montresor thinks Fortunato did, but we can assume it was nothing significant because he is not specific, and because Fortunato does not seem to be aware that there was an injustice done at all.

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that gave utterance to a threat.

Fortunato goes with Montresor down into the catacombs at night.  No one would do that if he felt that a person was out to get him for revenge.  You just do not go underground with people you have mortally insulted!

Yet Fortunato has no idea that he ever insulted Montresor or that Montresor is harboring a murderous rage because of it.

It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend. He accosted me with excessive warmth, for he had been drinking much.

Would you really show someone excessive warmth if you thought they were holding a grudge against you?  No, you would more likely be very cautious.  Montresor gets away with murder because Fortunato has no idea that he is even angry.

The hyperbole in saying that Fortunato committed a “thousand injuries” and the fact that Fortunato is not suspicious adds up to the idea that Fortunato did not really do anything.  Montresor imagined it.  There was probably some minor slight that no one else would have noticed, which Montresor blew out of proportion. 

Montresor is clearly a madman.  Madmen do not make very good friends.  He is having some kind of delusion about Fortunato, and because of that Fortunato has to die.

Montresor uses reverse psychology to trick Fortunado into going into the crypt with him by suggesting that he will have someone else look at the wine and by asking him if he is sick once he gets there.

Reverse psychology is the act of tricking someone into doing something by telling them not to.  Montresor uses this technique twice.  First he tells Fortunato that he will have someone else look at the Amontillado wine cask if he doesn’t.  Then he brings attention to his cough and tells him to leave.

Montresor’s ruse to get Fortunato into the crypt is asking him to look at a special cask of Amontillado wine.  He tells Fortunato that if he does not look at the wine, he will get someone else to do it instead.

“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”

“Luchesi cannot tell Amontillado from Sherry.”

“And yet some fools will have it that his taste is a match for your own.”

Montresor knows that Fortunato will never give up the chance to show off his wine knowledge.  By suggesting that he will show the wine to Luchesi, he is pretty much guaranteeing that Fortunato will come to see the wine.  He knows Fortunato very well.

Montresor uses the trick again to pretend that he wants Fortunato to go back once they are actually in the crypt.

“How long have you had that cough?”…

“It is nothing,” he said, at last.

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. ..."

This is actually pretty brilliant.  Fortunato is drunk, and does not see through the ruse.  He just insists that he is fine, and they continue.  By the time Fortunato figures out that Montresor actually does not care about his health and in fact wants to kill him it is too late!

The fact that Montresor is able to use reverse psychology so effectively shows that he is a good judge of character.  That he can lie so convincingly is further proof that he is a psychopath.  He wants revenge, and he will stop at nothing to get it.

There are many reasons that the author has Montresor keep suggesting that they go back. Perhaps the most important reason is that it will make Montresor seem perfectly harmless to Fortunato. If Montresor keeps suggesting going back, then he can't be leading him anywhere that could be dangerous.

But Montresor knows that Fortunato could easily become suspicious. Montresor is taking him a long, long way through a network of dark passages. Why on earth should he have stored a big barrel of wine so far away from the bottom of the stairs leading down into his wine cellar. The farther they go, the more strange it must seem. The "pipe," if it existed, would contain 126 gallons of wine. That is a huge barrel. The men would have had a very difficult time carrying it or rolling it through all those catacombs. It is only because Fortunato is heavily intoxicated that he doesn't protest. Poe describes his intoxication as follows:

He turned towards me, and looked into my eves with two filmy orbs that distilled the rheum of intoxication.

Poe had to have Montresor take Fortunato all the way from the street where he first encounters him back to his palazzo, down into the wine vault, and through a series of catacombs without saying anything about the Amontillado. It would seem natural for Fortunato, who is supposedly an expert, to ask questions such as "Where did you get it?" and "How much did you pay?" But Poe didn't want to Fortunato asking questions. Fortunato knows more about Amontillado than Montresor; otherwise Montresor wouldn't be asking his advice. If Fortunato started asking questions, he would probably sense that Montresor was lying. Poe himself may have known nothing about Amontillado except that it was a gourmet sherry and was an important export from Spain.

Instead of talking about that which is the object of their trip, the two men engage in chit-chat about the Masons, family crests, the nitre covering the walls of the catacombs, and other miscellaneous subjects. One of the ways Poe fills the gap with dialogue is to have Montresor keep suggesting that they go back. For example:

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”

Poe seems to be indicating that Montresor is using what is called "reverse psychology" to keep his victim motivated. But this is also a way of filling a lot of space with dialogue.

So Montresor keeps suggesting that they turn back because:

• Poe has to fill up some space with dialogue.
• The suggestions make Montresor look innocent. He can't be leading Fortunato into any danger if he tells him to turn back.
• Montresor is using "reverse psychology" because this is often an effective way to get some people--especially drunks--to insist on doing the opposite. We all know of drunks who insist on driving themselves home just because their friends are trying to talk them into letting someone else drive.
• The suggestion to turn back is a distraction. It is beginning to seem ridiculous that they should be walking so far in these bone-filled, stygian catacombs to find a single barrel of wine. Poe is not only distracting Fortunato but distracting the reader, keeping him from asking awkward questions, such as, "Why did you move the wine-barrel way back here?" "Why didn't you just tap the barrel for a couple of bottles and bring them up to your living room, where people could sample the wine in comfort?"

Montresor may be using reverse psychology in urging Fortunato to turn back, but he has another reason for doing so. Reverse psychology, so-called, plays upon the natural human tendency to do the opposite of what is recommended. This is especially observable in people who are heavily intoxicated. If told that they have had enough to drink, they will order another one. If told they should let somebody drive them home, they will insist on driving their car home by themselves. Reverse psychology may be hard to understand, but it is certainly easy to observe because it is so common.

However, Montresor is much more concerned about something other than using reverse psychology to lure Fortunato to his death. Montresor is leading the drunken man deep underground into a dark, dank, poisonous atmosphere. He doesn't want Fortunato to become suspicious or alarmed. By constantly telling his victim that they ought to turn back, Montresor is demonstrating that he has no evil intentions. He makes it seem as if he really doesn't want to take Fortunato to the place where he has stored the Amontillado. If he had any sinister motives, why would he keep urging Fortunato to go back upstairs? Montresor has been playing the naive innocent all along. He keeps up the pretence of pure innocence until the last moment.

In "The Cask of Amontillado," one of the conditions that Montesor sets for his revenge is that it "precluded the idea of risk, and that he [the avenger] must not only punish, but punish with impunity."  So, if Montesor does not lure Fortunato far enough into the catacombs, there is the risk that the self-professed connaisseur may escape or be heard if he screams. 

For the safety of his scheme, Montesor must get Fortunato into the deepest chambers.  If he does not distract him by playing upon his jealousy of Luchesi and keep him drunk along the way, Fortunato may realize that he is going deeper and deeper beneath the earth.  It is through his playing upon Fortunato's jealousy, his feigned concern, and his supplying of more wine to Fortunato that Montesor is able to get his victim to the recessed tomb designated for his revenge, a spot so far from the opening that no one can hear his screams until he finally dies.  (Do not forget that the man is buried alive.) 

Montressor knows that Fortunato will do virtually anything to imbibe in the supposed bottle of vintage Amontillado that he claims to have purchased. Fortunato's love of spirits is overwhelming.

He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. 

Under the pretense that he must quickly receive a second opinion about the quality of the nonexistent Amontillado, Montressor merely threatens to allow Luchesi to sample it instead of Fortunato, knowing that the man's curiosity will be too strong to pass up this opportunity. But it is simply a lure--a ruse--to keep Fortunato interested. By pretending to go back, Montressor makes Fortunato even more curious: so curious that he is willing to risk his health--and, unknowingly, his life--to make the descent into the catacombs. 

You are absolutely right to say that Montresor wants to kill Fortunato.  But he keeps giving him chances to leave precisely because he does want to kill him.  It's what they call reverse psychology -- he knows that giving Fortunato those chances to leave will make him much more determined to come further.

If you look at the times he gives Fortunato the chances to leave, he usually says that he'll get Luchesi instead.  When he says that, he is playing on Fortunato's pride.  He knows that Fortunato will never leave because if he did, Montresor would go get his big rival.

So Montresor keeps giving Fortunato chances to leave, but he does it because he knows that it will actually make Fortunato less likely to leave.

In "The Cask of Amontillado," Montresor uses verbal manipulations and nonverbal strategies to lure Fortunato to his unfortunate end. Montresor has studied his enemy well and knows his weak points. He has also picked an ideal night for his crime.

Understanding that Fortunato thinks of himself as an expert in wines, Montresor lies and tells Fortunato that he has a rare kind of sherry called amontillado in his wine cellar. He knows this will excite Fortunato's interest. It does, especially when Montresor says he is not sure it is the real thing. Fortunato becomes more and more interested in wanting to go down into the catacombs immediately and taste the amontillado for himself to make a determination. Then, Montresor heightens Fortunato's desires even more when says he will ask Luchesi, a rival, to do the sampling, as Fortunato is busy. Fortunato, who has the vanity to think he is the true expert, eagerly insists he is fully available.

Montresor uses more reverse psychology to trap Fortunato, saying it is too damp down in the catacombs for Fortunato to go there and that nitre, or mold, is growing on the walls. However, the more obstacles Montresor puts in his path, the more Fortunato is determined to head for the catacombs, which is exactly what Montresor wants, despite all his protestations of fake concern for the health of his "friend," such as saying "the vaults are insufferably damp."

By calling him "friend" and flattering him with concern, Montresor causes Fortunato's guard to lower. Fortunato has no reason to suspect that Montresor would do him harm.

Even as they are making their way through the catacombs, Montresor continues to use manipulative verbal reverse psychology and flattery, pretending he has Fortunato's best interest at heart. For example, after Fortunato has a coughing fit, Montresor says,

"Come," I said, with decision, "we will go back; your health is precious. You are rich, respected, admired, beloved."

Yet the more Montresor tries to deter him, the more Fortunato insists on going forward.

An important nonverbal strategy Montresor uses is to pick the night of Carnival, when Fortunato is drinking and partying, to approach him. Fortunato is in a happy space, deep into enjoying the festivities. A second nonverbal strategy Montresor uses is to keep Fortunato drinking as they travel through the catacombs. This way, Fortunato cannot think straight, making it easier for him to miss signs of danger.

A third nonverbal strategy Montresor uses is to wear a black silk face mask. This would be expected during a carnival, but it serves the purpose of hiding any facial expression that might tip Fortunato off to danger.

Montresor has planned his terrible crime well, using the verbal lure of a fine wine, flattery, playing on Fortunato's vanity, reverse psychology, and then, nonverbally, the setting of Carnival, alcohol, and his mask to disguise what he is doing from his enemy and wall him up in the catacombs to perish.

Many first-time readers might not understand the subtlety in Montresor’s method of luring Fortunato into his catacombs. They believe Fortunato is motivated by a desire (1) to drink some delicious Amontillado, (2) to demonstrate his connoisseurship, (3) to do Montresor a favor, and (4) to prove he knows more about wine than Luchesi. None of these beliefs is entirely correct. The whole story does not have to be read to understand how Montresor has baited his trap. The following contains all the information necessary to appreciate the thought Poe devoted to fashioning the story Montresor tells Fortunato. The nonexistent cask of Amontillado is the bait. The first minutes are crucial.

I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”

“How?” said he. “Amontillado. A pipe? Impossible! And in the middle of the carnival!”

“I have my doubts,” I replied; “and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain.”

“Amontillado!”

“I have my doubts.”

“Amontillado!”

“And I must satisfy them.”

“Amontillado!”

“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”

Montresor says, “You are luckily met” because he wants Fortunato to think he has been seeking him on an important matter. Then Montresor says, “But I have received…” The words “But” and “received” deserve particular attention. “But” implies that Montresor would like to join Fortunato, but he is on an urgent errand. He does not say he has purchased a pipe of Amontillado, only that he has “received” one. This suggests he has previously ordered the cask that has just been delivered. He has to say “received” so that Fortunato will believe the wine is at Montresor’s home and nowhere else. The obvious assumption is that a ship from Barcelona has arrived with an entire cargo of Amontillado. There is no other way Montresor’s cask could have been transported to Venice.

Fortunato does not ask questions about the transaction for several reasons, including that he is drunk. The “But” gives Fortunato no time to ask questions of a man in a hurry. And he does not want to show too much interest in details for fear of revealing that he would like to get in on this bargain. He must volunteer to sample the wine before Montresor goes to Luchesi. If he accompanies Montresor to his palazzo, Fortunato can keep Luchesi from finding out about this bargain. What interests Fortunato is the possibility of making a huge profit, and not sipping a glass of sweet wine in a cold, dark, damp catacomb in order to please a friend and to show off his supposed connoisseurship. Fortunato would not have to go with Montresor at all if it were not for Luchesi, who would also be very interested in the bargain if he learned about it. Fortunato otherwise could tell Montresor he could not accompany him that night—after all, he is inadequately dressed, he has a bad cold, and he could invent a previous engagement—and then go directly to the harbor and find the Spanish ship. He doesn’t need to taste Montresor’s wine; he can sample it from several big casks on board.

Why does Montresor repeat, “I have my doubts”? The manifest meaning is that he needs an expert to advise him. But why so urgently? Because he got a bargain and wants to buy more before word gets out. That is why he is running around looking for Fortunato and then giving up on finding him and heading for Luchesi’s. He wants to buy more wine that night, but he has to be sure it is genuine. Otherwise it is no bargain. But Poe concocted another reason for “I have my doubts.” If Fortunato cannot accompany him that night, he is sure to inquire about it later. This is one reason Montresor says he “received” the pipe. He can say he bought it from a person who wishes to remain anonymous. He never claimed there was more available or that he wanted to buy more. And if Fortunato asks to taste that totally fictitious Amontillado, Montresor can present him with a bottle of ordinary sherry and say it was drawn from the big cask. Fortunato will taste it, shake his head, and hopefully forget the matter. Montresor will have to think of some other way of disposing of his enemy. He will never be able to lure him into his catacombs with a similar cock-and-bull story.

Finally Montresor says, “As you are engaged...” He wants to find out whether Fortunato is expected at home or anywhere else that night. He must leave a cold trail. He doesn’t want Fortunato missed before tomorrow morning at the earliest. Montresor fails to respond to the first gambit. But when Fortunato takes him by the arm and proceeds to drag him off to his wine cellar, Montresor says:

“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—"

“I have no engagement,—come.”

Once Fortunato says, “I have no engagement,” his doom is sealed. Fortunato will be recognized by many drunken celebrants, especially with his conspicuous jester’s costume and jingling bells; but his companion, wearing a black cloak and a black mask, will be as nameless as a shadow.

The subtlety of Montresor’s entrapment scheme could be lost on first-time readers, who might make the assumption that Fortunato is motivated by a desire (1) to drink some delicious Amontillado, (2) to demonstrate his connoisseurship, (3) to do his friend a favor, and (4) to prove he knows more about wine than Luchesi. But a careful reader will see much more in the story, which can be read over and over with new insights.

At the beginning of the story, Montresor does not provide details or elaborate on specific incidents for why he plots revenge on Fortunato, but does mention that Fortunato had wrong him a thousand times and insulted his family's prestigious name. Montresor then tells the reader that he has plotted revenge for some time and mentions that he must punish Fortunato without being caught. Montresor then says,

"A wrong is unredressed when retribution overtakes its redresser" (Poe, 1).

Essentially, Montresor means that a wrong is not straightened or atoned for when the punishment overtakes the person enacting revenge.Montresor believes that it is essential to avoid being punished while enacting revenge upon a person. As the story progresses, Montresor deceives Fortunato by acting amiably toward him before leading him down into his family's catacombs, where Montresor shackles and buries Fortunato alive. At the end of the story, Montresor reveals that he successfully enacted revenge without being caught and Fortunato has been buried in his catacombs for a half of century. 

In Edgar Allan Poe's "The Cask of Amontillado," the narrator, like so many of Poe's narrators, is unreliable in that he does not provide any reason for his revenge other than the vague statement,

The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult,I vowed revenge.

Then, ironically, the narrator assumes that readers know him: 

You who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.

Obviously, then, there is a great deal of ambiguity about Montresor's "revenge" that he feels (1) he must seek and perform with impunity as well as (2) receive acknowlegement of this revenge on the part of the victim.  Both of these goals of revenge are attained:  Montresor walls in Fortunato in the tomb/catacombs, and Fortunato is well aware of what Montresor has done as he calls to him, asking to be allowed to return to the carnival, and finally crying "For the love of God, Montresor!"

We do not know why Montresor wants to have revenge on Fortunato.  We are told that Fortunato has "injured" him in a thousand ways.  But we also can see that Fortunato does not fear Montresor or act strange around him.  So that implies that Fortunato does not know Montresor is mad at him.

The phrase that you cite means that there you do not really get revenge if you get caught and punished for doing it.  So Montresor does not think it is real revenge to kill someone if you get caught and punished for the murder.

Montresor describes his chaining of Fortunato in fairly precise detail. There is a very narrow niche in the granite wall of the catacombs. 

Within the wall thus exposed by the displacing of the bones, we perceived a still interior crypt or recess, in depth about four feet, in width three, in height six or seven. . . . In its surface were two iron staples, distant from each other about two feet, horizontally. From one of these depended a short chain, from the other a padlock. Throwing the links about his waist, it was but the work of a few seconds to secure it.

Montresor did not install these chains. They had been there for centuries and used by feudal lords to dispose of disobedient vassals, who would simply disappear and never be heard from again. Montresor must have gotten the idea of killing Fortunato this way while exploring the catacombs. Concealing the crypt or recess with a stone wall was his own innovation. Since it is only three feet wide and six or seven feet high, it will be easy to build the wall.

One chain is only two feet long. The other is only a stub of a chain with a padlock. Montresor only has to draw the longer chain across Fortunato's body and secure it with the padlock. The recess is four feet deep, so the victim will be unable to interfere with the construction of the wall or knock it down after it is finished but still damp. His arms would only be about three feet long, so the wall would be a full foot beyond his reach.

He is pinned tightly to the wall. He has no chance of slipping out from under the chains or of climbing over them. Montresor has specified that Fortunato is wearing a tight-fitting jester's costume. He has no flexibility in such an outfit, and he probably doesn't even have any pockets. He has no weapon and nothing he could use as a tool to try to file a chain or pick the padlock. The padlock is old-fashioned. It is locked with a key rather than snapping shut like our modern padlocks.

Withdrawing the key I stepped back from the recess.

Fortunato would find it impossible to pick such a padlock even if he had some sort of tool. His death will be agonizing because he will have to stand in one upright position until he dies of starvation. Montresor repeatedly specifies that there is a lot of dripping water, so Fortunato will be able to lick drops off his hands and off the rock wall. It takes much longer for a man to die of starvation than of thirst.

Montresor has an easy time chaining Fortunato because of the simplicity. He also has an easy time building the stone wall because it only needs to be three feet wide. He is not using bricks but stones. The stones are heavy and may be almost a full foot long, so three or four stones would make up a whole tier. He has already prepared the mortar (because Poe wants this part of the story to end quickly); and the mortar stays damp because of all the dripping water. Montresor keeps the trowel on his person because he doesn't want it to rust. At the same time, he doesn't want to have to go someplace to fetch it when he brings Fortunato home. He uses the remaining mortar he has mixed in the trough to "plaster over" the entire stone wall and make it look like part of the catacomb wall itself. Then he covers the fake wall with what he calls a "rampart" of human bones. He is writing or telling this story fifty years after the event, and in all that time no one has discovered what is left of Fortunato.

Throughout the story Montresor offers several descriptions of Fortunato that offer some insight into his reasons for wanting him dead. At the beginning of the story Montresor describes Fortunato as "a man to be respected and feared" although he has one major weak point. Montresor also describes Fortunato's knowledge and connoisseurship of paintings, gems, and wine. Although Fortunato's knowledge of wine is his strength, his love of wine (along with his susceptibility to flattery) is also his weakness. Montresor meets Fortunato late at night when he is already very drunk and convinces him to come down to his family's vaults to try a cask of rare Amontillado wine. Montresor convinces him through a mix of his love of wine, his pride, and his susceptibility to flattery to follow him, suggesting that without the help of Fortunato (or one of his rivals Luchesi) he will be unable to determine if the Amontillado is real. Once they go down into the vaults Montresor keeps Fortunato distracted with a mixture of wine and continued flattery. He flatters Fortunato saying that,

You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter.

This description of Fortunato serves to keep him distracted from Montresor's true plan, but also suggests some of Montresor's reasons for wanting him dead. Fortunato is successful, popular, and happy in a way that Montresor is not and he cannot bare this.

He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian MILLIONAIRES. In painting and gemmary, Fortunato, like his countrymen , was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.

This is the third paragraph of "The Cask of Amontillado" quoted in full. It conveys two main ideas. One is that Montresor is not an Italian but a Frenchman. He distances himself from the Italians by his disparagement of their judgment of "painting and gemmary." His family may have lived in Venice for several centuries, but he is still an outsider as far as the upper-class Italians are concerned. His catacombs may be full of human bones--but these are not necessarily all bones of his ancestors. He may not have cleared out the bones that did not belong to his family because there was no place to put them; or there may be some law against such removal. In fact, it is quite possible that Montresor doesn't even own his palazzo but is renting it. He is obviously not affluent. He says that he bought largely (of Italian wines) "whenever I could." That should be interpreted to mean whenever he could affordto. There would always be plenty of Italian wines available.

The other main idea conveyed in Poe's third paragraph is that both Montresor and Fortunato are specialists in luxury goods and must both be earning their livings through buying and selling expensive merchandise to wealthy people. They are in the same line of business, but they are also competitors. While Montresor is poor, Fortunato is rich. It is very likely that the "thousand injuries" which Montresor does not explain are injuries suffered in business deals. Fortunato can outbid him. Fortunato can afford to buy in larger quantities. And Fortunato has family connections going back for over a thousand years. Italians would favor him because of his family status. If there is something good to be bought at a bargain price, Fortunato is more likely to hear about it before Montresor.

When Montresor tells Fortunato that he has bought a cask of Amontillado, Fortunato says, "Impossible!" What he really means is that it would be impossible for Montresor to learn about such a cargo of valluable wine before he did. But this is carnival season and Fortunato has been drinking and not attending to business. He thinks this is why Montresor has gotten ahead of him. However, Montresor has only bought one "pipe" (126 gallons) because, as he says, "I have my doubts" (about the genuineness of the wine). Fortunato is highly motivated to taste it--not because he needs any more wine, not because he is anxious to show off his connoisseurship, not to accommodate a friend--but because he wants to buy some of the Amontillado himself for resale. But he himself must taste it to make sure it is genuine, since Montresor has repeatedly expressed his doubts. Fortunato can afford to buy the whole cargo and make a big profit--and Montresor knows that is exactly what Fortunato is planning to do because that is exactly the sort of injurious thing Fortunato has done in the past.

Montresor tells Fortunato that he has bought some wine that he thinks is Amontillado, but he needs an expert opinion to tell him if it is genuine. Montresor flatters Fortunato in order to get him down in the catacombs. He knows Fortunato considers himself an expert of wines, so he uses that to lure him to his death.

Montresor shows his cleverness in several places throughout "The Cask of Amontillado." When he first encounters Fortunato celebrating in the street, he wants to make sure that he is not expected anywhere that night. He would like to lure Fortunato to his palazzo and leave a cold trail. If Montresor's victim were expected at home, for example, then relatives, friends, and servants might go out looking for him that very night, and there would be many people who would remember seeing him and who would at least be able to tell in which direction he was going. But if Fortunato were not missed until the following morning, everybody would be sleeping or hung over and would not remember anything of use. So Montresor cleverly ascertains that Fortunato is not expected anywhere by pretending to believe he is expected. 

“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”

“Luchesi cannot tell Amontillado from Sherry.”

Montresor does not get the information he wants, but at least he plants the idea that he is on his way to Luchesi and suggests that he is in a big hurry to have his Amontillado judged by a connoisseur. So he tries again.

“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”

“I have no engagement;—come.”

That was what Montresor wanted to find out. Fortunato will not be missed anywhere that night. Even tomorrow his family will assume that he is sleeping it off at a friend's home, or spending the night with a mistress. It will be some little time before people start wondering what on earth could have happened to Fortunato.

Montresor shows his cleverness by his use of "reverse psychology," a method of persuasion by telling a person to do the opposite of what you want them to do. Montresor must realize that Fortunato, as drunk as he is, will begin to wonder why the big "pipe" of wine is stored at such a distance from the foot of the cellar stairs. Here is an example of Montresor's reverse psychology.

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”

“Enough,” he said; “the cough's a mere nothing; it will not kill me. I shall not die of a cough.”

There is a natural human tendency to resist being told what to do. This is especially observable in drunks. By pretending to want Fortunato to turn back, Montresor suggests that there could be no danger ahead and that he could not have any ulterior motive for leading Fortunato onward.

There are many other examples of Montresor's cleverness. For instance, he has honed his entrapment story so that it will be nearly foolproof. He says he bought a cask of Amontillado without consulting an expert because he was afraid of losing a bargain. It is the bargain that interests Fortunato and not the prospect of sipping a glass of wine in a dank underground setting when he has a bad cold and is not adequately dressed for it. If Montresor got a pipe of 126 gallons of gourmet sherry at a bargain price, then Fortunato would like to buy some himself and sell it at a profit. Montresor knows his man. He understands that Fortunato is only going with him to his palazzo in order to prevent him from going to Luchesi. Fortunato assumes there must be a Spanish ship in the harbor carrying a cargo of Amontillado and offering it at a bargain price because it is hard to find buyers during the carnival when everybody is neglecting business. Montresor has been injured by his friendly enemy a thousand times, and he knows Fortunato is planning to tell him his nonexistent wine is only ordinary sherry, whether it is or not; then, if it is genuine, go to find the nonexistent Spanish ship and buy up the whole cargo.

One word--pride.  Fortunato has apparently slighted Montressor (the narrator) to the point where he feels must take the ultimate revenge.  He says there have been a "thousand injuries" which he has overlooked; but Fortunato insulted him, an act which could not go unpunished. The plan is simple, devious, and cruel.  Montressor will lure the offender to the catacombs at his palazzo (the underground burial place for his dead ancestors)  and then bury him alive.

But, this plan only works if he is able to lure Fortunato to his vaults. Montressor is apparently a student of human nature, and he uses his knowledge to carry out his plan.  The story takes place during Carnival, festivities similar to Mardi Gras.  He tells his servants he will be gone until the morning but they are NOT to leave--knowing full well they will go join the festivities once they know he is gone.  Thus, an empty palazzo.

Next, Montressor finds the slightly drunk Fortunato and tells him he has bought a large quantity (a full pipe) of Amontillado.  This is unlikely, since it's the middle of Carnival, and both men express their doubts as to the authenticity of the wine.  Montressor has studied his victim and begins to set the trap, appealing to Fortunato's pride in being the only true connoisseur of wine.

Montressor invites him to come and be the judge, which he rather reluctantly agrees to do--as he has a bit of a cold and he'd rather be part of the festivites.  Each time Fortunato appears to hesitate or falter, Montressor invokes the name of a rival, Luchesi, to lure him back.  When Fortunato insists on tasting the amontillado, Montressor says, "My friend, no; I will not impose upon your good nature.  I perceive you have an engagement.  Luchesi--"  The implication, of course, is that Luchesi would be able to do the job just as well. Each time the name is invoked, Fortunato is even more adamant that he be taken to the wine cellar to see for himself.  "As for Luchesi," says Fortunato, "he cannot distinguish between Sherry and Amontillado."

Thus it continues...all the way down to the catacombs of the palazzo.  Montressor says Fortunato should turn back because it's damp and he doesn't want his friend to catch pneumonia, and Fortunato says he is stronger than that.  Pride again.  Montressor offers to save him the trouble and get Luchesi, and Fortunato is even more adamant about going.  Even at the end, Montressor acts like a caring friend, unwilling to have Fortunato harmed or injured in any way, while Fortunato nearly forces his way into his own grave.

The plan was just too easy.  It hinged on the knowledge the narrator had of Fortunato's pride and the consistency with which he used that pride against his enemy.  Montressor's definition of revenge is that he must not be punished for his act of avenging and the "avenger must make himself felt as such to him who has done the wrong."  The great irony, of course, is that Fortunato did not, even at the end, understand why Montressor would do such a thing to him--calling into question both the insult by Fortunato and the excessive pride of his murderer.

The narrator in this story is a man named Montressor who hates Fortunato.  He lures Fortunato to his death by playing on Fortunato's vanity.

What he tells Fortunato is that he (Montressor) has this new pipe of wine.  He thinks that it is an amontillado.  But he tells Fortunato that he is not sure.  Fortunato is very proud of his knowledge of wines and so he wants to come and see whether the wine really is amontillado.

Along the way, Montressor plays on Fortunato's pride and competitiveness by saying he can get another man to come and assess the wine just as well.  He says Fortunato is too sick and should go home.  This makes Fortunato want to go even more.

In Edgar Allan Poe’s 1846 short story “The Cask of Amontillado,” the narrator, Montresor, seeks revenge against Fortunato because of the latter’s repeated instances of verbal abuse at the former’s expense:

“The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge."

Montresor’s plan for vengeance involves luring Fortunato to the former’s home, although the form Montresor’s revenge will take remains a mystery until the end.  In order to convince his intended victim to cooperate and walk of his own free will toward whatever fate lay in store, Montresor appeals to Fortunato’s vanity and knowledge of and affection for fine wines.  As Poe has his protagonist explain,

In Poe's "Cask of Amontillado," Montressor's idea of perfect revenge is confessing to us that he murdered Fortunato over fifty years ago and knowing that he has gotten away with it and there's nothing anyone can do to punish him for it.  In short, the idea worked as the perfect muderand the perfect murder story.

Not only was Montressor's plan brilliant, luring Fortunato to his catacombs using a bottle of amontillado as a red herring, but his execution of the plan shows bravado.  He foreshadows his vengeful intentions to Fortunato several times on the passage down to the niche using the trowel, the coat of arms, and much verbal irony.  But Fortunato is blinded and drunk by greed and wine, playing right into Montressor's trap.

Montressor's plan to bury Fortunato alive in an underground vault also shows much forethought.  No one would think to look for him there; much less, there could be little proof of a murder, since Fortunato's body is walled up and mixed with other corpses' bones.

The real brilliance of the revenge, though, comes in being able to retell it over fifty years after.  Either Montressor is a brilliant mastermind or a lunatic who invented the whole story.  Either way, as an old man, it takes a masterful mind to remember/invent such a horror story knowing that he has impunity.

Montresor uses Fortunato's pride to entice him to go and try the Amontillado. Montresor notes Fortunato's weakness in the third paragraph: 

He had a weak point--this Fortunato--although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. 

Knowing this, Montresor intended to exact his revenge on Fortunato by exploiting that weakness. He first mentions that he has the Amontillado but has his doubts about it, encouraging Fortunato to brag about his connoisseurship. Montresor then adds that he's going to ask Luchesi to sample the Amontillado in order to give him a judgment on it. Montresor knows Fortunato will say that he is the more seasoned wine taster. Fortunato adds, "And as for Luchesi, he cannot distinguish Sherry from Amontillado." 

As they go further in to the catacombs, Montresor continuously suggests that they go back to save Fortunato's health and nagging cough. Fortunato, in his pride, takes this as a dare, as if Montresor is daring him to continue on. Fortunato urges them to continue on; thus, Montresor makes Fortunato the captain of the journey that will lead to his end (Fortunato's). 

Montresor shows that he is obviously a sadist and that he is enjoying the success of the revenge plan he has been working on for a long time. In addition, Poe needed to fill up some time. After all, building a stone wall from floor to ceiling is not a simple task. Poe didn't want to spend too much time describing the arduous job of building the wall, but he didn't want to make it look too easy either. The real climax to the story occurs when Montresor snaps the padlock that secures the chains around Fortunato's waist. After that, as a good story teller, Poe wanted to conclude the tale neatly and adroitly. However, there was still the big job of laying tier after tier of stones, plastering over the wall with more of the same mortar, and covering the whole section as thoroughly as possible with human bones. Poe's story-telling genius can be seen in the way he manages to cover all this work in so few sentences. When Montresor takes a break from his work, it provides a break from the repetitious description of working with the mortar and stones;, and it also creates the illusion that more time went into the project without wasting additional words. The reader might well wonder how Montresor became so proficient as a mason that he was able to create a nearly perfect wall that appeared to be just a part of the natural granite walls of the catacombs. Montresor wanted to be sure that at no time in the future would anyone who might happen to be down there might notice the existence of an artificial wall and wonder what was behind it. Poe makes the task seem simple. But how many of his readers would be capable of doing such a perfect job--unless, of course, a reader might be a professional mason?

At this point in the story, Montresor has clubbed Fortunado on the head, knocking him out, and chained him to a wall. As Fortunado slowly regains consciousness, Montresor begins laying bricks to trap Fortunado. He pauses periodically when Fortunado moans in pain; Fortunado eventually wakes up and begins screaming.

I reapproached the wall; I replied to the yells of him who clamoured. I re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamourer grew still.

This implies that Montresor is actively seeking to enjoy and mock Fortunado's predicament.

Thus Montresor is, by all means, a sadist; someone who takes pleasure in inflicting pain upon others. Were he merely concerned with exacting revenge upon Fortunado, he would simply brick him up in the catacomb and be done with it; yet he keeps pausing, checking, reassuring himself, and so on, all with the purpose of knowing that Fortunado has no chance of escape, and that Fortunado is aware of the long and miserable death that awaits him. In fact, it seems odd that Montresor doesn't take the opportunity to reveal to Fortunado exactly why he is doing this; the fact that Fortunado doesn't ask either lends some credence to Montresor's earlier claim that Fortunado's insult was the last straw that pushed him to commit this act.

Montresor is trying to trick Fortunato. He wants to lure him into the cellar, on the face of it to taste a cask of the finest amontillado. But of course his plans are more sinister; he wants to kill Fortunato for various (unspecified) wrongs. To make his ruse more effective, it is necessary for Montresor to give the impression that he likes Fortunato and wants to be friends with him. He also wants to make it seem as if he really doesn't want Fortunato to try the wine—that he's trying to dissuade him from venturing down into the cellar:

My friend, no. It is not the engagement, but the severe cold with which I perceive you are afflicted. The vaults are insufferably damp. They are encrusted with nitre.

Montresor is being very sneaky here. He knows that Fortunato will be more curious about the wine if he senses that Montresor is trying to discourage him from visiting the cellar. He also plays on Fortunato's vanity as a wine connoisseur. Montresor strategically says he wouldn't want to put Fortunato to any trouble; plus, it will be so horribly cold and damp down there. He says maybe he should ask Luchresi for his opinion instead, to which Fortunato replies:

Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchresi, he cannot distinguish Sherry from Amontillado.

The ruse works. And tragically for Fortunato, he comes to regret being tricked by Montresor.

Edgar Allan Poe must have spent a long time thinking about how any man could lure an enemy underground and murder him without being seen in his company and without being suspected of any complicity after the victim disappeared. It is easy enough to think of telling Fortunato that he has a cask of gourmet wine he wants him to sample. But he has to get Fortunato to come with him immediately. Otherwise Fortunato could find out that there was no Spanish ship in the harbor which had just brought in a cargo of Amontillado. Montresor does not exactly tell Fortunato that this is how he "received" the wine, but Fortunato would assume that was the case because it was the only likely way that a big cask of Amontillado sherry come from Barcelona to Venice. Montresor has to tell Fortunato that he got a bargain and that he is going to Luchesi, since he had been unable to find Fortunato.

It is the  bargain that gives Fortunato such a strong motivation to sample the wine immediately.Unlike Montresor, Fortunato is a rich man and could buy up the whole shipload. Otherwise, he probably would have put Montresor off. He was having fun at the carnival. He was not adequately dressed for going into cold, damp catacombs. He was drunk. He had a cold. It is the bargain and his fear that Luchesi will beat him out of it that motivates him to go home with Montresor immediately.

Montresor knows that Fortunato is thinking of buying up the whole cargo. He wouldn't have to sample Montresor's cask if the Spanish ship really existed. He could beg off for that night and then easily find the ship in the harbor and sample the wine on board. Then he could bargain with the captain for the entire cargo, assuming it was genuine. But he has to sample Montresor's (nonexistent) Amontillado in order to keep him from going to Luchesi, who never appears in the story but is apparently a wealthy man who buys and sells expensive items for resale, just like Montresor and Fortunato.

So Montresor invents a clever story to deceive Fortunato into coming with him immediately. The whole entrapment story is compressed into these few brief paragraphs of dialogue.

I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”

“I have my doubts,” I replied; “and I was silly enough to pay the full Amontillado price without consulting you in the matter. You were not to be found, and I was fearful of losing a bargain.”

“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”

Montresor acts as if he is in a big hurry to get a connoisseur to judge his wine. This can only suggest that he wants to buy more while it is still being offered at a bargain price. But he doesn't dare to buy any more until he is sure it is genuine Amontillado.

Montresor knows that Fortunato is planning to tell him his (nonexistent) wine is only ordinary sherry, thereby eliminating Montresor as a competitive buyer as well as Luchesi. This explains why Fortunato doesn't ask a lot of awkward questions about the wine. He doesn't want to show too keen an interest.

When Montresor gets Fortunato down into his catacombs he deceives him further by suggesting more than once that they turn back because of the risk to his victim's health. For example:

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”

This deceit is of secondary importance. Montresor uses reverse psychology to keep Fortunato moving towards the niche where he intends to chain him to the granite wall. But Montresor has a rapier and Fortunato, in his tight-fitting jester's costume, is obviously unarmed. Montresor could kill him any time he wants. The victim is as good as dead already. The really important deception is contained in the totally false, finely honed story Montresor has concocted to lure Fortunato to his palazzo and down the steep steps into his doom.

We know that Montresor is a proud man when he says that the reason he kills Fortunato is because "he ventured on insult". In other words, once Fortunato insulted Montresor, Fortunato could not forgive him and he decided to reek revenge on poor Fortunato. The persuasive techinique Montresor uses are also very ingenious. He appeals to Fortunato's ego when he keeps repeating, "Oh, there is always Luchesi". Fortunato considers himself an expert on fine wine and is not about to let Luchesi or anyone else verify whether Montresor's wine is Amontillado or not. Finally, the idea of revenge is present throughout the story. At the beginning, Montresor says that "redress must not overtake the redressor. I must punish[ Fortunato] and punish him with impunity." Montresor has planned his revenge very carefully so he will not get caught. As he takes Fortunato a further and further into his vaults, Fortunato makes the sign of a mason, a selective and secretive group of businessmen. When Montresor doesn't understand the sign because he's not a mason, Montresor says, "Yes. I'm a mason." and pulls out a trowel, a tool of a common mason. This shows he plans to wall up Fortunato all along and it was not simply a whim.

Initially, Montresor comments that in order to successfully carry out revenge one must "not only punish but punish with impunity." He begins by maintaining an affable relationship with Fortunato by continually smiling during their conversations, which allows him to hide his true malevolent feelings. Montresor puts his plan into action after running into a rather intoxicated Fortunato during the "supreme madness of the carnival season." Knowing that Fortunato considers himself to be a connoisseur of wine and is an arrogant man, Montresor cleverly baits Fortunato by telling him that he has a pipe of Amontillado and that he plans on asking Luchresi to verify its authenticity. Montresor knows that Fortunato will criticize Luchresi and volunteer to taste the wine. Montresor then takes Fortunato to his empty palazzo, where there are no servants or attendants. Montresor explains to his audience that he had specifically told his servants that he would not be home later, knowing that they would abscond from their duties.

Once Fortunato arrives at Montresor's home, Montresor leads him through the depths of his underground catacombs. Several times Montresor offers Fortunato wine along their journey to further intoxicate him while he insincerely encourages Fortunato to turn back. Montresor knows that Fortunato's pride will motivate him to continue traveling deeper until they reach the Amontillado. When Fortunato reaches the end of the vault, Montresor quickly shackles his arms and begins building a wall behind Fortunato. Brick by brick, Montresor buries Fortunato alive in the depths of his family's catacombs, where no one will hear or discover him. Montresor then refuses to reveal his secret for fifty years.

Montresor, Poe’s narrator, demonstrates his superior knowledge of human psychology when he manipulates Fortunato’s ego. Montresor claims that Fortunato “knew a lot about fine wines, and proudly believed that he was a trained judge of them.” Here the reader begins to see Montresor’s knowledge of human psychology revealed; he knows to play on Fortunato’s pride. Montresor does not invite Fortunato to verify his wine but says that he will get Luchresi, a rival, to do so. This plays against Fortunato’s pride and causes him to follow Montresor. Montresor continues to demonstrate an understanding of human psychology when he fakes concern for Fortunato’s health. Montresor says, “But I can see you are trembling with the cold. Come! Let us go back before it is too late.” Fortunato plays down his own concerns and sickness to be contrary to Montresor. This manipulation of Fortunato through false concern ensures that Montresor’s plan will continue. He also plays on Fortunato’s fondness for wine, coaxing him further down the vault with the thoughts of an “Amontillado.” Montresor demonstrates an understanding of human psychology by manipulating Fortunato, playing to his pride, faking concern for his health, and using his desire for fine wine.

Though Montresor’s manipulation of Fortunato is a good example of his superior knowledge of human psychology, it is not the only example. To ensure that there would be no witnesses, he had to get his servants out for the evening. To do so, he notes, “I had told the servants that they must not leave the palace, as I would not return until the following morning and they must care for the place. This, I knew, was enough to make it certain that they would all leave as soon as my back was turned.” Montresor set up the conditions that he knew would result in the servants leaving.

Montresor begins the story by saying, "The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge."  Montresor employs a figure of speech called hyperbole, also referred to as overstatement, to emphasize just how incredibly offended and insulted he felt by Fortunato.  It is unlikely that Fortunato had actually injured him a "thousand" times, but Montresor felt as though he had.  This, in part, motivates his need for revenge.

Further, Montresor says that his family motto is "Nemo me impune lacessit," which translates to "You will not harm me with impunity."  This means that no one can harm a Montresor without being punished or repaid for it.  Thus, if Montresor believes that he has been harmed by Fortunato, his family pride will not permit him to allow it to go unavenged.  He clearly possesses such a sense of family pride, as he states that "The Montresors were a great and numerous family."

Therefore, what motivates him to him to seek revenge on Fortunato is a mixture of wounded personal pride and family honor. 

Montresor never makes specific what Fortunato has done to him, suggesting that the "thousand injuries" he may or may not have suffered are less important than Montresor's perception of injury. 

We do get a hint that Fortunato considers himself superior to Montresor. When Montresor says he is a mason, Fortunato takes this to mean a Freemason, a group which apparently Fortunato is a part of--and Forunato exclaims "You! Impossible!" He "recoils" when Montresor shows him the trowel, and says "You jest." Then his mind returns to his quest for the Amontillado, and he urges Montresor on.

This dialogue, though short and merely suggestive, speaks volumes in a compact story. It hints that Fortunato considers that he is lowering himself to be with Montresor, a man he cannot seriously consider a fellow Mason, and only condescends to do so because of the bait of the rare wine.

On the other hand, Fortunato's snobbery is not stated directly as fact by either of the characters. One of the most interesting facets of this story is its porousness, the way it leaves itself open to construction and interpretation. 

Poe actually never has Montresor state the specific reasons behind his wanting revenge, though he hints at several possibilities. Although he does mention injuries and insults, he never reveals details.

First, Montresor seems vindictive and almost paranoid. It could well be that the wrongs for which he is obtaining revenge never actually happened and are products of Montresor's imagination. 

Another possibility is that Montresor comes from a wealthy family now on hard times. He may blame Fortunato for the change in his family's fortunes or may simply resent Fortunato for being a nouveau riche.

Both men are wine connoisseurs and possibly merchants. There appears to be some rivalry concerning expertise in wines, or perhaps there was some sort of commercial rivalry in the past.

Part of the point of of this vagueness is to convey the idea that the desire for vengeance has become so overwhelming that the original cause no longer matters; instead, the vengeance and its planning have become an obsession for Montresor.

Fortunato is too drunk to stop Montresor and doesn't realize he is about to be murdered.

Fortunato never really tries to stop Montesor, because he doesn’t realize what is happening.  This is because he is very drunk.  The murder happens during the Carnival holiday.  Fortunato is dressed up and drinking.  Montresor tells him he needs information about a rare wine. 

Fortunato falls for it, because he has a big ego.  When Montresor pretends that he is going to ask someone else about the wine, that gets Fotunato even more interested.  It never occurs to him that it is a trick.

"As you are engaged, I am on my way to Luchresi. If any one has a critical turn it is he. He will tell me --"

"Luchresi cannot tell Amontillado from Sherry."

This little bit of reverse psychology continues when Montresor pretends to care about Fortunato’s health.  He says he is coughing and they should turn back.  Fortunato doesn’t want to.  He laughs at Montresor’s trowel, again not realizing anything is up.  Montresor makes a jokes about Masons.

Fortunato doesn’t realize what is happening until he is actually being bricked into the wall.  The drunkenness wears off and he starts to panic.  Montresor is aware of this.

A succession of loud and shrill screams, bursting suddenly from the throat of the chained form, seemed to thrust me violently back. For a brief moment I hesitated, I trembled. Unsheathing my rapier, I began to grope with it about the recess; but the thought of an instant reassured me. 

It is no use by this time.  Fortunato never has a chance to stop Montresor. Montresor makes sure that Fortunato suffocates to death.  He remains there and in fifty years no one finds him.   Montresor has gotten his revenge for whatever imagined injuries Fortunato did to him.

Montresor begins his narrative by stating

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.

Since Montresor never describes the insult or any of the injuries, readers have offered all sorts of conflicting opinions about his motivation. Some believe that Montresor was never injured or insulted at all and therefore must be insane. Poe avoids having to clarify or justify Montresor's motivation by having him address his communication, or confession, or letter, to a person he calls 

You, who so well know the nature of my soul

Presumably this person knows so much about Montresor that it is not necessary for him to give any examples of the injuries. This device is effective because it forces the reader to pay close attention to the text in an effort to deduce facts that are fully understood by "You, who so well know the nature of my soul." This is very much like Ernest Hemingway's famous "Iceberg principle." 

If a writer of prose knows enough of what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of an ice-berg is due to only one-eighth of it being above water. A writer who omits things because he does not know them only makes hollow places in his writing.
                         Ernest Hemingway, Death in the Afternoon

It does not seem likely that Montresor could be insane and write such a coherent narrative. He must be sane and telling the exact truth. There should be indications within the narrative of the types of injuries that have driven Montresor to plan and execute his revenge. The injuries would have to be of a kind that are known only to Montresor and Fortunato. Montresor wants to kill with "impunity," and he could not expect impunity if it were widely known that he had been injured by Fortunato anything like a thousand times. Poe offers a clue in the third paragraph of the story, which should be read with special attention.

He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.

This sounds as if both men are "gentlemen-brokers" who earn their livings by dealing in luxury goods such as paintings, jewelry,antiques, gourmet wines--even in real estate. Fortunato is rich, Montresor is poor. Montresor puts up with Fortunato's injuries because he is dependent on him financially. There are many impoverished Venetian aristocrats who are forced to dispose of family heirlooms in order to survive in their decaying palazzi. Montresor may often need to borrow from the man he constantly describes as his good friend in order to purchase an item for resale. Or he may go into an ad hoc partnership with Fortunato on a purchase. Or he may simply receive a finder's fee for introducing his good friend to a prospective buyer or seller of some one-of-a-kind family treasure. Fortunato would have plenty of opportunities to "injure" Montresor, without anyone else knowing about it, by taking an unfair share of the profits on a transaction, by paying a lower finder's fee than agreed upon, or cheating him in a dozen other ways. As Montresor says to his good friend when they are in the catacombs:

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter."

Montresor knows that Fortunato is planning to cheat him on the nonexistent Amontillado. It would be just the sort of opportunity Fortunato could not resist. He is planning to taste the wine and, assuming it is genuine, tell Montresor it is only ordinary sherry, then find the nonexistent Spanish ship with its nonexistent cargo of Amontillado and buy up the whole shipload. When Montresor found out what happened, Fortunato would laugh it off as "an excellent jest." He is a scoundrel, but he considers himself a funny fellow, which is why he wears a jester's costume in the carnival.

This is a logical explanation of why Montresor, who is extremely clever and perfectly sane, should want revenge against Fortunato. Montresor has been cheated by this man so many times that he knows he can entrap him simply by offering him an apparent opportunity to cheat him once again.

In "The Cask of Amontillado" the focus falls upon two characters, that of Montressor and Fortunato. Montresor has sworn revenge on Fortunato and to "not only punish, but punish with impunity". Fortunato is characterized as being very vain. Montresor then uses this weakness to his advantage to fulfill his motives of revenge. "He had a weak point -  this Fortunato - . . . he prided himself upon his connoisseurship in wine." Montresor uses this to his advantage by appealing to his vanity and enticing him through the appeal of using his expertise in wine tasting. He lures Fortunato to his vaults and, ultimately, to his demise. He tauntingly uses the other unseen character Luchesi, to bait Fortunato on by comparing their ability in the realm of wine tasting. This strikes Fortunato's vanity and encourages him to continue to prove that he is the real expert. Another trait that makes Fortunato so easily preyed upon is his trusting belief that Montresor has no ill will toward him. Montresor states that "neither by word nor deed had I given Fortunato cause to doubt my good will." Therefore, Fortunato remains trusting and self-centered in his desire to prove his commitment to the Amontillado.

Even though we hear from our narrator, Montresor, regarding a lifetime of insults and humiliation that have been heaped upon him by the unfortunate Fortunato, we do not experience this behavior during their encounter in the story. What we see is the drunk Fortunato warmly greeting Montresor as an old friend. Because of his knowledge of Fortunato, Montresor is able to approach Fortunato as a friend without incurring any suspicion from Fortunato. Montresor then exploits this trust and his knowledge of Fortunato's character to murder him. Montresor plays upon Fortunato's knowledge of wine and his pride in being a wine connoisseur in order to entice him to follow him deeper and deeper into the catacombs beneath Montresor's home. He then uses Fortunato's disdain for another man, Luchesi, and his wine-tasting abilities to keep Fortunato walking with him deeper into the catacombs.

While walking, Montresor again exploits their friendship by expressing concern for Fortunato's health as his coughing becomes worse within the damp catacombs, though this is also a ruse. He knows Fortunato's personality and conviction will not allow him to turn back. Fortunato's implicit trust of Montresor is exploited to the point where the reader begins to question the reliability of the narrator and whether or not Fortunato deserves his truly unfortunate end.

In "The Cask of Amontillado," by Edgar Allan Poe, Montresor's motivation for killing Fortunato is simple: he was insulted. In the opening line of the story, Montresor (narrator) explains that he suffered through many hurts imposed by Fortunato, but would not put up with an insult: "The thousand injuries of Fortunato I had borned as I best could, but when he ventured upon insult I vowed revenge," (1).

It is unclear what the insult was regarding, but one might guess it had something to do with familial protection, as family seems important to Montresor, (illustrated in his discussion of his family's coat of arms); or it may have to do with wine connoisseurship as both men seem to pride themselves on this skill; "I was skillful in the Italian vintages myself, and bought largely whenever I could," (2).

Interestingly, the family motto of Montresor is "Nemo me impune lacessit," meaning "No one attacks me with impunity." This implies that members of the Montresor family do not allow anyone to hurt them and then get away with it. However, Montresor's plan to wall Fortunato up inside the catacombs without anyone ever knowing about it is an illustration of exactly that. In fact, Montresor is never punished for his deed because he's never found out: "For the half of a century no mortal has disturbed [his bones]," (6).

It is that he has no bona fide cause for revenge (one that he states) that makes this story so horrifying.  By not identifying the cause for revenge, we cannot dismiss it as unworthy or outside of our range of experience.  Instead, the revenge achieves a universality, a flaw of human character, or perhaps more aptly, a commonality of human emotion that Montressor works out for us that we, only in our unconscious, would dare to think about.  Poe often treats such taboo subjects that he considers fundamental to human experience but too awful for a person to lay claim to.  Indeed, in an essay on poetry he calls such horrific experiences and emotions "beautiful."

Unfortunately, the answer to your question is no. Montresor does not detail his reason for revenge other than to say that Fortunato had "done him a thousand injuries". He doesn't detail any single one, nor does he ever give any specific reason for his cruel revenge. It could be assumed that, as in many of Poe's tales, there is not actually any wrong deed done to lead to the horrible action, but there is nothing in writing to prove that. 

One of the most commonly accepted interpretations of this question is that Montresor is facing his own death.  There are many hints to this, both at the beginning and the end.  Keep in mind, before I give the clues, that Montresor is an adult man when the scene with Fortunato occurs - adult enough to be master on his own home and servants.

Already an adult, Montresor tells us in the last paragraph, that the event he has described came 50 years before:

For the half of a century no mortal has disturbed them. In pace requiescat!

This would make Montresor somewhere between 75 and 100 years old - perhaps sick and facing his own death and needing to tell his story.

Now understanding how old he is, we go back over the story and find that Montresor was addressing someone who knew his soul:

"you who so well know the nature of my soul"

Typically, the soul is the purvey of a religious scholar.  The assumption is that Montresor, facing death, is confessing either to a priest or to God directly in the hopes of absolving himself of his sin before he passes on.

Montresor spends his energy primarily manipulating Fortunato.

First, when Montresor meets Fortunato, knowing that the other man loves fine wines, he hooks the other man's interest by saying that he has bought a rare cask (container) of amontillado. He appeals to Fortunato superior skills as a connoisseur to accompany him so that Fortunato can tell Montresor whether he has been tricked in the transaction.

The second way Montresor manipulates Fortunato is by pretending that he is willing to ask Luchesi (another connoisseur of fine wines) to check the amontillado if Fortunato is too busy. Fortunato's ego will not allow him to consider letting Montresor share the wine with anyone else, and so he insists upon going with Montresor.

A third way Montresor manipulates Fortunato is by repeatedly asking after his health as they move deeper into the catacombs beneath Montresor's palazzo. Montresor acts concerned and asks many times if Fortunato will not return above ground for the sake of his health, but he also gives Fortunato more to drink. Under these circumstances, and the repeated references to the amontillado, Fortunato refuses to leave.

In these ways, Montresor convinces Fortunato to willingly proceed to his own death.

For the most part,"The Cask of Amontillado" involves only two characters: Montresor, who vows to be avenged for "the thousand injuries of Fortunato" and his victim, Fortunato, who fancies himself a connossieur of wines.  In his elaborate plan to lure Fortunato into his family catacombs in order to murder him, Montresor devises methods of luring, or manipulating, Fortunato:

1. Montresor tells Fortunato that he has recently acquired a "pipe" of the Amontillado wine, but he has his "doubts."  When Fortunato hesitates to judge it for Montresor, Montresor says that he is on his way to Luchesi:  "If anyone has a critical turn, it is he.  He will tell me---" Hearing the name of Luchesi, Fortunato becomes jealous:  "Luchesi cannot tell Amontillado from sherry."  Montresor continues to play upon this professional jealousy by telling Fortunato that he sees that Fortunato has "an engagement," but Luchesi maybe can come.  Enraged, Fortunato declares, "I have no engagement--come."  They proceed.
2. As they approach the catacombs of the Montresors, Fortunato staggers some while Montresor points out the niter and remarks that Fortunato has a bad cough and they should not proceed:  "You are a man to be missed....Besides, there is Luchesi--"  Again, this touches the nerve of professional jealousy in Fortunato:  "Enough,...the cough is a mere nothing," Fortunato protests. And they proceed.
3. After giving the already drunk Fortunato more wine, Montresor lures him farther and farther into the catacombs.  In vain Fortunato tries to see into the depths, but Montresor manipulates him yet again by saying "Proceed,...herein is the Amontillado.  As for Luchesi--" 

"He is an ignoramus," interrupts the drunk and still jealous Fortunato who desires to be the first to judge the Amontillado.    

As Fortunato exclaims, "The Amontillado!" Montresor begins to wall up the opening to the niche and wreak his vengeance that he has so long planned.

In his efforts to redress the "thousand injuries" that he feels Fortunato has committed against him, Montresor lays a careful plan to ensure that his retribution is performed with "impunity":

1. He ensures that Fortunato will not be missed by planning his revenge at the time of the "carnival," which is the celebration of Mardi Gras, the day before Lent. People are in masques and costumes and celebrating with libation, so the absence of Fortunato is one that will not easily be observed.
2. Montresor tempts Fortunato's excessive pride by telling him, "As you are engaged, I am on my way to Luchresi.  If any one has a critical turn, it is he. He will tell me--" In jealousy, Fortunato replies, "Luchresi cannot tell Amontillado frm Sherry."
3. Montresor manipulates Fortunato, urging him not to enter the catacombs because he has a cough and the niter is damaging to the health, knowing full well that the man's pride compels him to enter. Frequently, too, Montresor acts as though he is concerned fot the man's health:  "...we will go back;you will be ill, and I cannot be responsible.  Besides, there is Luchresi-----"

Poe has Montresor address his confidential communication to a single individual whom he calls "You, who so well know the nature of my soul." This is a brilliant literary device and foreshadows Ernest Hemingway's "iceberg theory," which also involved leaving out exposition. In doing this, Poe relieves himself of the need for providing a whole lot of exposition and can focus on the actual crime, the dramatic action, itself. Poe does not even have to mention where the events took place. Some readers actually think the crime might have occurred in New Orleans! Or perhaps in Kansas City, Missouri? Poe does not have to describe the "thousand injuries of Fortunato" because, presumably, he has told this confidant, or confidante, all about his relations with Fortunato before, presumably in letters.

I believe it is a mistake to assume that Montresor wants revenge because of some unspecified "insult." In my opinion it is the "thousand injuries" for which he wants his revenge. The insult, whatever it was, only shows that Fortunato was becoming more obnoxious and insufferable, and would continue to infuriate Montresor in the future.

Why does Montresor put up with a thousand injuries? Why does he associate with Fortunato at all? There is no need for Montresor to explain this to "You, who so well know the nature of my soul," but we readers have to play Sherlock Holmes and deduce from the existing evidence. Here is the third paragraph of the story in full:

He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.

This suggests that Montresor and Fortunato deal in luxury items. They are not Italian noblemen but gentlemen-brokers and connoisseurs. Fortunato is rich. Montresor is poor. Montresor may depend on him for loans, finder's fees, ad hoc partnerships in single transactions, and other such financial benefits. Fortunato may have cheated Montresor many times or taken unfair advantage of him in other ways. Since these matters would be between the two men, Montresor could continue to speak of Fortunato as "my friend," "my old friend," "my good friend," and "my best friend," in order to convince everyone, including Fortunato himself, that they were good friends and thereby forestall incurring any possible suspicion when Fortunato turns up missing. Montresor has so thoroughly conditioned himself to thinking of Fortunato as his friend that he refers to him constantly as such throughout the story, even when he is taking him to the niche where he intends to chain him to the granite wall and leave him to die of starvation and madness.

Fortunato will not die for a single insult but for a thousand past injuries. 

There really isn't a reason ever given for Montressor's hatred of Fortunado. This is what makes the tale all the more compelling and puzzling.

Some critics feel that Fortunado embodies all that is repugnant about the middle class aspiring to be elite. Fortunado's snobbishniss about wines, for example, illustrates this principle. He may be the unfortunate, physical target of Montressor's animosity.

Others feel that there simply is no basis other than the fact that Montressor is crazy. The catacombs may represent and the descent into darkness reflects his own warped thinking and pure evil.

I think you mean Montresor instead of Fortunato. I'm not sure what you mean by "is it necessary", but from Montresor's point of view, it is very necessary. The story is told in first person point of view, and Montresor makes his case to the reader why he feels it's necessary. Also consider his family motto and coat of arms--anyone who does harm to the Montresors will be punished. He feels it's also his family duty to punish Fortunato. The offense incurred by Fortunato doesn't matter because he's done something that makes Montresor think that Fortunato has wronged him. The fact that Montresor feels offended is all that's necessary for him to believe Fortunato must be punished. Whether he's insane or not is left up to the reader, but he certainly spends time trying to convince us he isn't mad.