"The Cask of Amontillado" is a story of revenge, but the reader is never told exactly what Fortunato did to warrant such vengeance. In fact, throughout the story, the reader gradually realizes that Montresor is an unreliable narrator; that whatever insult Montresor believes Fortunato committed is probably imagined or exaggerated. It's certain that Fortunato has no idea of Montresor's anger, and this makes the story even more tragic and frightening. The seemingly happy jangling of the bells on the top of Fortunato's cap become more and more sad the deeper the two venture into the catacombs.
In the beginning of the story, Montresor defines revenge. He says he must "punish with impunity." He states if the avenger is caught, or does not make the punishment known to he who committed the wrong, the wrong goes unavenged.
With this in mind, he sets the trap for Fortunato. He gives Fortunato numerous opportunities to back out, using the tricks of classic conmen by playing on Fortunato's greed and pride. In fact, it is Fortunato who insists they carry on to find the Amontillado, and this will no doubt torture him as he is buried alive. Montresor also provides hints as to what he plans to do with Fortunato. He seemingly miraculously comes up with a cask of Amontillado during carnival, which Fortunato can scarcely believe. He tells Fortunato, "You are a man to be missed," and after Fortunato says he won't die of a cough, Montresor agrees. His family motto is "No one insults me with impunity" and he is carrying a trowel. Yet Fortunato suspects nothing, and is so shocked when Montresor chains him to the wall, he doesn't even try to fight.
The structure of the story places the events 50 years in the past. Montresor, perhaps on his own deathbed, is telling someone, perhaps a priest, the story, but not with any remorse. He still believes Fortunato wronged him, and at the end eerily says "In pace requiescat," or "May he rest in peace."