Montresor, the narrator of Edgar Allan Poe's short story "The Cask of Amontillado," has quite clearly lost his grip on reality. He has become obsessed with the insult Fortunato has given to him and is committed to revenge. This obsession and vow now control his whole life. He spends all of his time planning what he will do to Fortunato in every detail, covering every possible circumstance and making sure that no one will ever know what he has done. He wants Fortunato to feel this revenge in the extreme, but he also wants to escape punishment for it.
Montresor seems somewhat sane as the story progresses, but readers know that he cannot be, for in his obsession, he has lost touch with the reality that Fortunato is a human being. Montresor carries out his plan with deliberateness, making sure that no detail is overlooked. He pretends to be taking Fortunato into the vaults to inspect a cask of Amontillado and leads him through the vaults until they reach the innermost compartment. Here he chains Fortunato to the wall and proceeds, with a calm purposefulness, to seal up the room.
Montresor works steadily until Fortunato begins to scream. Suddenly, he, too, begins to tremble and then scream along with Fortunato, revealing in this act that his madness is floating toward the surface of his mind. As Montresor finishes his job and turns to leave, Fortunato, now mostly crazy himself, laughs hilariously, hoping beyond hope that Montresor's actions are just a joke. Then he falls silent, his mind apparently shattered.
Montresor himself feels his heart sicken but blames it on the dampness of the vaults. He puts the last stone in place, his revenge now complete but his own mind plunged deeply into psychosis, for he has just left a human being chained to a wall in a dark vault to die a horrible death. Sane people are less likely to do this.