In Edgar Allan Poe's short story, "The Cask of Amontillado," the narrator and protagonist, Montresor, is found to be mentally unbalanced or deranged to some extent. Though Poe never directly states that Montresor is insane, that fact is made evident by the same character's actions and speech. While the information given in the opening paragraph is enough to suggest to the reader that Montresor is not well, the contents of the rest of the story offer many pieces of evidence that tell the reader that Montresor's mental illness is a certainty.
In the first paragraph of the story, Montresor divulges his requirements for revenge and states that, should his plan fail to result in meeting each of the prerequisites, he would not have been vendicated at all. The extreme nature of his views on his vendetta indicate that Montresor is "off."
As Fortunato is led to the scene of his imprisonment and, ultimately, death, Montresor frequently mentions his concern for Fortunato's health while also luring him to continue onward.
"Come," said I, with decision. "we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi--"
Perhaps the most telling evidence of Montresor's deranged state occurs after he becomes startled by the screams coming from Fortunato as he is being entombed. After his initial fright, which leads Montresor to attempt to stab Fortunato, Montresor calms himself, reapproaches the partially sealed compartment, and screams back at Fortunato so loudly that Fortunato's cries are overpowered.
...I reapproached the wall; I replied to the yells of him who clamored. I reechoed, I aided, I surpassed them in volume and in strength...
Clearly, these are not the actions of a mentally stable person.