What was Fortunato's crime?
As Poe's "The Cask of Amantillado" begins, Montressor says:
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult.
During the story, however, Montressor neither divulges any of the injuries nor the insult. Since Montressor is Italian and has a coat of arms whose symbol and motto are symbolic of revenge, the reader must assume that Montressor comes from an honor culture which prides itself on revenge in response to any individual or familial insults. The crime could have been something as trivial as a breech of manners; regardless, it is so negligible that Fortunato never suspects any offense, even when he finally realizes his doom.
Just as Iago never gives a good enough reason to take revenge on Othello, so too does Montressor never divulge his motivation. In this way, he is a vice character who prides himself on duping and taking advantage against his supposed enemies purely out of spite. It's a kind of game.
The reader must admit that Fortunato's crime during the story is drunkenness and gullibility. He arrives at Montressor's catacombs inebriated and with a bad cough. To venture far into the vaults, given the nitre, is foolish. In this way, he puts his health in jeopardy.
His main crime is materialism: he must have the amontillado. It is a rare possession that he, a connoisseur, must have, at all costs. To put his life in danger for the sake of a vintage wine violates the cardinal sins of pride and envy.
I do not think there is any way to say what Fortunato did to Montresor except to guess based on your own feelings about what might make you mad enough to do this.
Montresor never tells us what Fortunato has done to make him mad. He only says that there were a "thousand injuries." I do not think that any of the "injuries" could possibly have been very bad. The reason that I say this is that Fortunato had no idea that Montresor hated him. So it could not be, for example, that Fortunato had stolen Montresor's wife or anything big like that.
So I think maybe Fortunato hurt Montresor's feelings. Maybe he had said bad things about the quality of Montresor's wine? That might make the "punishment" be sort of poetic justice.