Nothing, Montresor was an insane unstable character.
The story does not say why Fortunato insulted Montresor. In fact, there is doubt that there ever were any insults at all. They might have simply been a product of Montresor's imagination
Caught at it or failed to have Fortunato know he is being punished. Early in the story Montresor states: " 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's underlying reason for plotting against Fortunato is likely his thirst for revenge, possibly due to a perceived wrongdoing or insult from Fortunato. Feelings of betrayal, jealousy, or a desire for power may also play a role in motivating Montresor's actions.
Montresor may be carrying a trowel to help him with sealing up the wall where he plans to entomb Fortunato. It serves as a tool for him to complete his act of revenge and ensure that Fortunato is trapped behind the bricks.
Montresor is an unreliable narrator because there are indications that he is either insane or the possessor of an overactive imagination. The indications come from Montresor's words and Fortunato's actions. Montresor begins the story by telling that he has suffered a thousand injuries at the hand of Fortunato , but now Fortunato has also insulted him. For that, Montresor seeks revenge and plans to murder Fortunato. The problem is that Montresor gives no details of these injuries or insults, leaving the reader to wonder if he is imagining all of it. Another indication that no such behavior ever occurred is found in Fortunato's behavior when he runs into Montresor at the carnival. Fortunato is friendly and gracious. He offers to leave the carnival to help Montresor test this Amontillado Montresor says he might have. In the cellar Fortunato suspects nothing untoward from Montresor even though they discuss Montresor's family motto which is that they will not allow anyone to insult the family. Surely, if Fortunato had acted so badly toward Montresor he would not have offered his help and would not have been so blind to the possibility that Montresor would uphold the family honor deep down in the cellar where Montresor could exact revenge with no one knowing about it. And that is just what happens.
The hook is knowing that Montresor plans on exacting the perfect revenge on Fortunato and reading further to see if he actually does it. Montresor states in the opening that a wrong is not avenged unless the avenger is not caught and that the person who committed the offense must know he is now paying for that offense. The reader gets hooked into continuing into the story to see if Montresor will actually commit the perfect crime and how he will do it.
In the opening line, Montresor states that he has suffered a thousand injuries at the hand of Fortunato but now Fortunato has also insulted him and this is too much to take, so Montresor plots revenge. There is an indication that Montresor is insane because there is doubt that Fortunato has ever injured or insulted Montresor at all. Montresor does not describe a single incident of injury nor does he describe the insult that supposedly has pushed him over the edge. Why would Montresor suffer a thousand injuries but plot revenge after only an insult? Aren't injuries more serious than insults? Wouldn't sticks and stones break Montresor's bones but words can never hurt him? Why plan to murder someone after an insult but not after all those injuries? Later when they meet at the carnival, Fortunato is very friendly toward Montresor. He hardly acts toward Montresor like he has injured him a thousand times and has just recently insulted him. Wouldn't Fortunato have said something to Montresor about the insult or about all those injuries he has laid on him in the past? Darn right, he would have, but he doesn't. Fortunato acts like they are the best of friends. Not only does Fortunato act friendly, but then volunteers to leave the carnival to go with Montresor to his home to test the Amontillado. Sure, Fortunato's vanity in being a connoisseur is part of that willingness, but surely, he would not have been so cheerful in doing Montresor such a favor. Finally, Fortunato, this supposed enemy of Montreesor goes down into Montresor's cellar without the slightest bit of trepidation about being all alone in the dark cellar with someone he has supposedly injured and insulted. Fortunato's action completely belie the idea that he is an enemy of Montresor. The one conclusion the reader can draw from the contradiction between Montresor's words and Fortunato's actions is that the injuries and insults Montresor mentioned are all in his head and that he is quite insane.
In 'The Cask of Amontillado,' Montresor provides numerous clues that had Fortunato been paying attention, might have seemed strange. First of all, he is continuously making references to Fortunato's health and the dangers of wet cold conditions. Obviously, this hints at the condition, Fortunato will soon find himself in. Next, when they discuss the Freemasons, Fortunato doubts Montresor is a member. He isn't even curious about why Montresor just happens to have a trowel on his person, which he reveals to prove his membership. When Montresor goes into a diatribe about his family coat of arms, Fortunato is either too drunk to pay heed.
Montresor stopped walling up Fortunato because he feared that someone might hear the commotion and discover his crime. The vibrations of the chains could have been a sign of Fortunato trying to break free or attract attention, prompting Montresor to act quickly and finish the job.
Not knowing what Fortunato did to Montresor heightens the horror of the story because the reader is never certain if Fortunato ever did anything wrong against Montresor that was deserving of such revenge. If Fortunato's so-called injuries and insults had been laid out for the reader to see, the reader would either sympathize with Montresor or with Fortunato depending on the reader's own point of view. Not knowing what was ever done, in deed if anything had even been done, leaves every reader wondering if Fortunato's death is just a terrible mistake by a madman.
Montresor's motive, as a character in the story, for telling of this event is not made clear. Some critics postulated that he is confessing his crime to a friend or even a priest, but there is nothing in the story itself to prove this. As a literary device, however, the use of the 'flashback' manner of telling a story can be effective to bring out the character's feeling and perception of the events in the story. For one thing, the only way the reader is assured that Montresor has exacted the perfect revenge is by a flashback. Montresor states in the opening that a wrong is not avenged unless the avenger is not caught and the person who has committed the offense knows he is paying for it now. If the story took place in present time, the reader would never know whether Fortunato somehow escaped his fate or whether Montresor got away with it. But, with Montresor telling the story fifty years later and confirming that the wall behind which Fortunato now rests has been undisturbed for half a century, it is clear that Fortunato is dead and that Montresor got away with it.
Montresor wants to make sure Fortunato is intoxicated so that he is easier to manipulate and lead into the catacombs where the betrayal will take place. Additionally, the wine serves as a way to lure Fortunato into following Montresor, making him believe he is in for a fun and enjoyable experience.