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.

In the famous horror story "The Cask of Amontillado" by Edgar Allen Poe, a rich nobleman named Montresor lures a man named Fortunato into the catacombs under his home with the promise of sampling some Amontillado wine. Once they have reached a remote chamber in the catacombs, Montresor chains Fortunato to the wall, bricks up the entrance, and leaves him there, entombed alive.

As readers, we would suppose that Fortunato must have done something terrible to Montresor to prompt such a horrifying act of revenge, but in fact, the only motivation that Poe mentions is in the first line. Montresor, the narrator, feels that Fortunato insulted him. Although he hints at other injuries, he does not delineate them, and he implies that he bore these other injuries without doing anything about them. He says:

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

Montresor's cunning, evil intent and lack of remorse throughout the story leads us to believe that he didn't perform this foul deed because Fortunato deserved it, but rather because of his own depraved nature.

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. 

Montresor wants to get revenge on Fortunado for some unnamed insult.

Montresor does not specify how he was insulted, or what the final insult was that got him so mad.

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

We do know that Montresor felt that he should be patient in getting his revenge, because revenge is not really revenge unless you escape punishment too.

Fortunado also suspected nothing.  This is another reason we are led to believe that the injury or insult was a minor one, and mostly in Montresor’s head.  After all, would you go underground to look at some wine if you thought you had insulted a man enough for him to want to kill you?

The simple answer to the question of why Montresor wants to kill Fortunato is that Montresor hates him. It doesn't matter whether Montresor is sane or insane, or whether he is justified or unjustified. He hates Fortunato so passionately that he wants to kill him in a horrible manner. Also, he wants to be sure that he doesn't get caught and punished. This is certainly understandable without a lot of explanation. Nobody would want to commit a murder and get sent to prison or executed. Montresor feels that he has been injured and wants revenge. He works his murder plot out carefully and is completely successful in every respect. When he tells the entire story in confidence to someone he addresses as "You, who so well know the nature of my soul," fifty years have passed. So it is obvious that he has achieved a perfect crime and that he has received the "closure" which was his main objective. His hatred for Fortunato tormented him. Once he has achieved his revenge he is cleansed of his tormenting feelings. That is why people seek revenge. He no longer feels any hatred for his victim. That is why he closes with the words, "In pace requiescat!" He means this sincerely. 

Montresor’s motive for leaving Fortunado in the catacombs is revenge for Fortunado’s insults against him.

Although he never tells us exactly what Fortunado did to him, Montresor opens the story with charging Fortunado with a “thousand injuries.”  Chances are these supposed insults were minor imaginings of a raving mind, because Montresor is not quite right in the head.

Montresor is obsessed with the perfect murder.  He is convinced that he is not really getting revenge unless he gets away with it.  If he is caught and punished, that will not really be sweet revenge.

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.

This is why Montresor chooses to brick Fortunado up in the catacombs, where no one will find him.  He will suffocate and die there, and no one will ever know that Montresor killed him.

The irony is that Fortunado has no idea that Montresor is even angry at him, so he doesn’t suspect anything.  There are no signs that he is going to his death.

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 tells the reader that Fortunato has done him a thousand injuries.

We are never actually told what Fortunato did.

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

It seems that Fortunato has no idea that Montresor is angry with him, or he would not have agreed to go underground.  Therefore the “injuries” are likely small and imagined, such as a supposed snub or unintentional rudeness interpreted from an innocent action.  Montresor makes it clear that he never uttered a threat or gave Fortunato cause for concern.  Instead, he pretended to be his friend and slowly planned the perfect murder.

Fortunato seems to be more highly placed in society than Montresor.  After all, he is a Mason and Montresor clearly is not (although he pretends to be).  Therefore the real reason Montresor kills him is probably because he is jealous and feels that he deserves what Fortunato has.

Montresor is a madman.  He could be driven by the slightest of actions, which is his mind becomes a huge injury that is not to be borne.  Fortunato did not stand a chance, because once Montresor's delusions set in he was a goner.

Montresor never admits to a clear motive for killing Fortunato, speaking only in the vaguest terms about it. First, he says he has suffered a "thousand injuries" from Fortunato. We have to assume this is hyperbole, which is figurative language based on exaggeration. Since we know that Montresor is telling the story many decades after the fact, he must have been a young man when he murdered Fortunato. How could there have possibly been a "thousand" injuries? The fact that Montresor exaggerates suggests to us that he has an inflated or unrealistic idea of how he has been wronged.

However, Montresor brushes off the thousand injuries and states that, in fact, it is being insulted that determines him to pursue the path of revenge. An injury is actual damage or harm that a person endures. An insult is being spoken to or treated with disrespect. We can conclude from this that Montresor is a very proud person, who has little toleration for being disrespected. We know, too, that later on, Fortunato will speak with disbelief at the idea that Montresor might be a mason, thinking Montresor means he is a member of the freemasons. Fortunato says: 

“You? Impossible! A mason?”

Montresor, however, means he is a literal mason who will wall Fortunato up to die.

It could be that Fortunato is simply a high-handed person who thinks he is better than Montresor, and Montresor, being mentally unstable, can't tolerate that. It could also be that he is walling up Fortunato due to some insult having to do with freemasonry. 

The important point is that whatever Fortunato has done or whatever Montresor perceives him to have done, walling him up to endure a slow, horrible death in the catacombs can't possibly be commensurate with whatever "crime" he committed.

Montresor is the protagonist and narrator of "The Cask of Amontillado" by Edgar Allan Poe, and he claims from the beginning that the reason he had to seek revenge is that he was insulted. We do not know until the story unfolds exactly what that means, but he expresses his motive clearly in the first line of the story: 

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

Here is the problem: Montresor is not a reliable narrator. What he says may have some truth in it, but we cannot trust his judgment. If the insult were serious enough to warrant being murdered, a "sane" person would not have been able to act as if nothing were wrong until he had the opportunity to arrange a murder.

It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued as was my wont, to smile in his face, and he did not perceive that my smile NOW was at the thought of his immolation.

If the insult were serious enough, the one who gave the insult (Fortunato) would not have been on such friendly terms with the man he insulted so strongly (Montresor). 

While it is clear that Montresor is perfectly capable of planning and executing a well-staged murder, he is not rational about his reason for doing so. He accepted "a thousand injuries" before this one insult, and yet the one insult was enough to prompt a murder. That is not rational thinking. 

Montresor is the narrator of Poe's short story The Cask of Amontillado. The reader does not learn that key bit of information until near the end of the story when Fortunato screams out and begs for his life.  

At the very beginning of the story, Montresor announces to his reader why Fortunato must die.  Montresor kills Fortunato out of revenge. More specifically, the reason for his revenge is that Fortunato insulted Montresor in some way.

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

Montresor never explains what those injuries were or what the insult was.  It is up to the reader to hypothesize a bunch of possible reasons. Fortunato is pompous and annoying for sure, but Montresor isn't exactly pure as the driven snow.  If he is killing somebody over a single insult, he is definitely a tense individual.  

Montresor does not go into detail about what specifically motivates him to murder Fortunato but does mention that Fortunato had caused him a "thousand injuries" before insulting him one last time. Fortunato is depicted as an arrogant, proud man, who openly insults individuals like Luchresi without thinking twice.

Montresor is portrayed as a vengeful individual, who takes pride in his heritage and family's impressive name. While Montresor leads the unsuspecting Fortunato into the depths of his catacombs, he mentions that his descendants were "great" and "numerous." He also spends time elaborating on his family's coat of arms and motto, which means "No one provokes me with impunity"—this emphasizes his terrifying plan of revenge.

Given Fortunato's propensity for insulting individuals and Montresor's family pride, one can assume that Fortunato more than likely insulted Montresor's family name, which motivates him to murder Fortunato by burying him alive in the vaults of his palazzo.

Montresor says he has suffered a "thousand" injuries from Fortunato but that, in the end, it is an insult that leads him to plot revenge. However, although he feels his pride has been damaged, Montresor never reveals exactly what the final, unforgivable insult was that led him to believe he needed to protect his pride and family honor through murdering another person in a horrendous way.

We have to wonder if the actual injuries and insult have gradually become less important to Montresor as he stuffs down his anger—smiling and pretending everything is fine—and then consumes himself with plotting vengeance. The revenge seems to have taken on a mad life of its own. One wonders what Fortunato could possibly have done that was so terrible that he deserves the fate of a slow death walled up in a catacomb.

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 never tells the reader specifically why he is seeking revenge on Fortunato. He only says that Fortunato has "inflicted a thousand insults". Montresor never tells the reader what those insults might have been which makes Montresor an unreliable narrator and therefore we should question the validity of his story, while cleverly plotted, it may not even be true.

This question has been previously answered, please see the link below.

The most puzzling and intriguing lines in "The Cask of Amontillado" are undoubtedly,

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

At this point, we reasonably expect Montresor to identify the nature of the "insult," particularly when it becomes clear that the insult, whatever it is, has driven Montresor to take such horrific revenge on Fortunato.  Fortunately, however, for many generations of readers and literary critics, Montresor leaves everyone mystified about the nature of the insult, which in turn casts doubt upon Montresor's reliability both as the narrator and as a person.  From a practical standpoint, if readers cannot trust Montresor-as-narrator, they can never be certain whether he is recounting events or personalities as they are or as he perceives them.  For example, our perception of Fortunato may not be accurate because we see him through the eyes of an unreliable (that is, obsessed) narrator, one of Poe's favorite character types.

One of the most important aspects of "The Cask of Amontillado," then, is not what we know about Montresor and his revenge, but what we do not know.  His failure to identify the insult, among other things, leads readers to debate endlessly about the cause of Montresor's revenge and, perhaps more important, gives the narrative an air of unresolvable mystery.  Without knowing the cause of Montresor's hatred, it becomes impossible for us to decide whether Montresor's revenge falls within justifiable behavior--assuming, of course, that anything justifies murder.

Montresor is vague about exactly how Fortunato has wronged him. So the reader can only guess as to whether Montresor was justified in his revenge. Considering the harsh nature of the murder, it seems unlikely that Montresor's crime can be justified at all, especially when Montresor claims that the last straw was a mere insult. Whatever injustice(s) Fortunato has allegedly done to Montresor, this is what led Montresor to exact his revenge on Fortunato. Montresor notes this in the first sentence of the story: 

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

Given this first line, we can only assume and suppose what the insult could have been. And if it indeed was just an insult, we might suppose that Montresor is too sensitive and/or had some other unknown motive for killing Fortunato. Whatever the insult was, Montresor became furious but calculating. Still in the first paragraph, he remarks: 

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. 

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.