In what ways does Montresor display his madness? Is it fair to consider him sadistic?in order to support a thesis statement
There are quite a few supporting factors to Montresor's madness. First he seeks and tricks Fortunato to follow him in the catacombs in order to bury him alive. He does this because of an insult, which he never identifies. This is not how a sane person reacts to being insulted, regardless of the insult.
Your thesis could read as follows: In Edgar Allen Poe’s “The Cask of Amontillado” Montresor demonstrates madness through his unstable thoughts as well as through his inappropriate reactions (or actions).
In this thesis you would have two main arguments – for the first you would argue that his reaction to being insulted is not sane and you can strengthen your arguments with how sane people react (psychology). And, your second argument would focus on how searching for someone at a carnival, getting him inebriated, and burying him alive is not the actions of a sane person.
Few sane people go to the extent of enacting revenge as Montressor. Although Fortunato's offense is never disclosed, it is unlikely that the act was deserving of his final reward. Pride plays a large part in the story, and Montressor takes extreme measures in avenging the slight to his name. The fact that he is willing to take the life of a man who claims to be his friend is a clue to his reduced grip on sanity, and his careful plotting and devious scheming to assure that there will be no witnesses to his crime is another example of his mental instability. The final act of walling Fortunato up to die a slow death is certainly a sadistic turn, and Montressor takes great pleasure in the horror that he knows his victim will endure.