One of the most interesting elements of Poe's "The Cask of Amontillado" is the issue of Montresor's mental state. For example, as justification for Montresor's hatred of Fortunato, we learn that
The thousand injuries of Fortunato I had borne as best I could but when he ventured upon insult, I vowed revenge.
From this point on, we continually wonder what "insult" could have angered Montresor--this is the first indication that we might be dealing with not only an unreliable narrator but also a mentally unbalanced narrator. The tension between what Montresor tells us, and what the truth is, builds as we realize the horrific death Montresor has planned for Fortunato. Montresor's reference to this unspecified "insult," then, is the first indication that all may not be right in his mind.
A second indication of Montresor's twisted state of mind is his proud boast of his own deviousness:
I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.
Whatever the insult was, it was enough to create an intense hatred within Montresor that could only be satisfied by Fortunato's being walled up alive in Montresor's catacombs. Rather than challenging Fortunato to a duel, which would have been standard practice among gentlemen of the time, Montresor engages in a very clever but morally depraved revenge plot and, more important, he seems to be proud of his devious plan. And the question all readers of this story have still goes unanswered: "What could Fortunato have done to create such an enemy?"
Even though Montresor is an intelligent, logical man--he plans and executes his revenge perfectly--his initial inability, or reluctance, to describe the "insult" casts a shadow over his mental state from the story's beginning to its end. Readers may admire the skill with which Montresor carries out his revenge, but most are convinced he's mentally unbalanced.