The story is told from Montresor's point-of-view, which makes him an unreliable narrator. Because he has exacted his twisted revenge and killed someone, we can assert that he is mentally ill in some way. Because the narrator is telling the story and discussing his own crimes, we get insight into his his motivations and lack of reason/logic for committing this crime. We are able to see how truly disturbing this crime is through the narrator's sense of calm and detachment about what he has done. He, remarkably, seems to have no remorse, nor does he seem to have any regrets whatsoever: " He tells the story from beginning to end with no diversion, no explanation, and no emotion. If he is gleeful at gaining his revenge, or if he feels guilty about his crime, he does not speak of it directly, and his language does not reveal it" (eNotes). He was bent on revenge and carried out his plan methodically.