Edgar Allan Poe, who wrote "The Cask of Amontillado," did an excellent job of conveying his narrator's unstable state of mind throughout the story. Poe's used first person narrative very cleverly and skillfully in order to lead the reader to create a clear mental impression of being told a story by a somewhat derange individual (Montresor, the narrator) who is obsessed with revenge, as well as being fully aware of his own cunning and success as a murderer.
The reader begins to perceive that Montresor is not quite "right" when he details his prerequisites for the act of revenge, which are found within the first paragraph. He quickly adds that his "smile now was at the thought of his immolation," which is not a particularly sane view of his nemesis. Montresor's pretense of concern for Fortunato's health, as well as his apparent pleasure at the naivete of his victim, are indicators of his lack of mental soundness.
Although Montresor seems to do a reasonably good job of maintaining a semblance of mental normalcy, there is one point in the story in which he appears to give in to his mind's illness:
...For a brief moment I hesitated, I trembled. Unsheathing my rapier, I began to grope with it about the recess; but the thought of an instant reassured me. I placed my hand upon the solid fabric of the catacombs, and felt satisfied. 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. I did this, and the clamorer grew still.
Although Montresor regains composure after his outburst, the reader is left with no doubt about his lack of mental stability.