Montresor's murder of Fortunato was premeditated, from the planning of an alibi to making sure the body is never found.
Montresor’s plan was to get revenge on Fortunato during the Carnival for some perceived insult. We have no idea what the insult was, but it was likely something minor because first of all Fortunato doesn’t suspect anything and second of all, Montresor is clearly deranged.
Montresor makes sure that all of his servants are out of the house, basically giving him an alibi because no one can say whether he was home or not.
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.
He knew that telling them this would “insure their immediate disappearance.” He chose a busy time when everyone would be out partying and no one would ask questions. Fortunato could disappear from whatever party he was attending and no one would really wonder where he was, because most people would probably be getting drunk and separated. Also, with people off in costumes no one would recognize either of them.
Montresor also stalked his prey. He knew enough about Fortunato to know that he loved wine, and that offering him wine would peak his interest. So he prepared the catacomb and told Fortunato that he had a very special wine that he needed to know about.
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.”
He knew that this would pique his interest. It is also enough of a story to get Fortunado underground, where wine might be stored. How else do you get a guy to go underground and allay suspicion?
Montresor leads Fortunato underground into the crypt, where the bones and wines are somehow kept side by side (yuck). In addition to asking for his advice, he also carefully gains his trust by pretending to care about his health. He makes jokes about being a freemason, giving a reason for having a trowel. All of these things, and Fortunado’s drunken weakened state, ensure that the man suspects nothing.
Montresor has prepared the scene before Fortunato gets there. Otherwise it would take too long. He has covered his materials with a pile of bones (they are in a crypt!) so it is not obvious when Fortunato first arrives. Montresor then just sort of guides him in.
In niche, and finding an instant he had reached the extremity of the niche, and finding his progress arrested by the rock, stood stupidly bewildered. A moment more and I had fettered him to the granite.
Then Montresor can brick the niche up. Fortunato is confused at first, and then gets more and more alarmed. He didn’t suspect anything, and didn’t know Montresor was a maniac. By the time he figures it out, he is a dead man and it is too late. Montresor is going to leave him down there.
In the end, he does say that his heart “grew sick.” Perhaps some doubts or spurts of conscience? Maybe. The end of the story is, “pace requiescat.” May he rest in peace. Yet Montresor cannot let him out. He needs to get away with the crime, and giving in now would be the end of him. No one will find the body down there.
This is a class Poe story of a madman narrator carefully explaining how clever he is at stalking and killing his prey. As with many of these killers, his only concern is that he not get caught, not guilt at what he has done. He wants us to appreciate his cleverness and the fact that Fortunato was asking for it and got what he deserved, for whatever unnamed insult. Notice that the insult is not named. The insult itself is not important, just getting away with the murder.
Montresor has had ample time to plot his seeming revenge. Bear in mind that while this story is read that Montresor is not a reliable narrator so we cannot even be certain that his revenge plot took place.
Needless to say he did plot out the perfect revenge. Fortunato genuinely believed that he and Montresor were good friends. When Fortunato sees Montresor at the carnival, Fortunato is drunk and Montresor is sober. Montresor tells us that he will use Fortunato's arrogance about his own wine connoisseurship against him. He makes up a story about having amontillado, then baits Fortunato by telling him he will have another "connoisseur" taste the rare sherry to test its genuineness.
Montresor then tells Fortunato that the wine is deep in the catacombs of his estate and Montresor has also taken care that his home is empty. He told his staff he would be out for the night and he has such distrust for his staff that he says as soon as they hear that they will take off without permission.
Finally, Montresor has all the materials (trowel, bricks, mortar) that he needs to bury Fortunato alive in the catacombs.
Montresor understands his prey very well. He approaches Fortunato when the man is at a carnival, in a good mood and already under the effects of alcohol. He tempts Fortunato with his favorite wine, and threatens to sell to another, which causes Fortunato to agree to go with Monstresor at that moment. Montresor has also made sure that his own house is empty, so that no one could see and interefere.
Montresor wants to murder Fortunato, but he stresses that he wants to do it with "impunity." He doesn't even want to be suspected of Fortunato's disappearance. That is Montresor's biggest problem. Throughout the narrative he continually refers to Fortunato as "my friend," "my poor friend," and "my good friend." Without explaining why he does this, he knows the reader will understand that he has been pretending friendship and calling Fortunato his good friend for so long that it has become a habit. When Fortunato's absence becomes a matter of deep concern, no one will think of suspecting Montresor because he is known to entertain such great affection for his friend.
Montresor waits for the big carnival to lure Fortunato to his palazzo. He knows that his victim will be drunk and not thinking clearly. Not only that, but everybody else will be drunk and not observing clearly. Montresor will be able to wear a black cloak and a black mask because everybody is in costume. No one could possibly recognize him. He has the stones and mortar prepared, and he is carrying the trowel under his cloak. He has made sure his servants will all be gone. He has plenty of wine down below which he can offer to Fortunato to keep him intoxicated and befuddled.
When he encounters Fortunato on the street he asks him, not once but twice, whether he is expected anywhere. If his victim was expected at his home or someone else's home momentarily, Montresor would have called off the execution and waited for another opportunity. He wants Fortunato's absence to go unnoticed for a good 24 hours, so that he will leave a cold trail. He explains that he has plenty of patience. He can live with his hatred as long as he knows that eventually he will take Fortunato's life. He says that he is always smiling at Fortunato in a friendly manner but that his smile is really at the thought of what he intends to do to him.
Montresor plots his revenge against Fortunato carefully, being certain
... 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...
Montresor chose to attack Fortunato's weak point: His love of sherry. He would use a rare bottle of Amontillado as a lure to draw Fortunato into his wine cellar. Montresor made sure none of his servants would be at his palazzo.
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.
Seeing that Fortunato was already drunk and would be less suspicious, Montresor led the man deep into the family catacombs, which doubled as a wine cellar. Montresor probably knew that the nitre encrusted on the walls would further weaken Fortunato, who had a bad cough. They drank several more bottles of wine along the way, and Fortunato became even more inebriated. Montresor made sure to bring a trowel within his roquelaire, and that Fortunato's final resting place would be deep within the catacombs, at an interior crypt with three walls. Montresor had made sure there were iron staples attached to the floor with a short chain and padlock. Under the pile of nearby bones were building stones and mortar, with which Montresor would construct a fourth wall--sealing Fortunato inside. It was a perfect crime, and
For the half of a century no mortal has disturbed them. In pace requiescat!