The narrator of Poe's story "The Tell-Tale Heart" is obviously insane, but that should not give anyone cause to think Montresor is also insane. In fact, it might be possible to strengthen the case that Montresor is sane by asserting that Poe would not be likely to write two murder stories in which he used the same kind of narrator. It should be noted that the narrator of "The Tell-Tale Heart" gives himself away and confesses, whereas Montresor keeps his secret for at least fifty years.
There is no reason to claim that Montresor is insane except for one element in his disclosure. He says that Fortunato has injured him "a thousand times," and yet he does not give a single example of an injury he has suffered. Some readers have concluded that Montresor has not suffered any injuries and therefore must be delusional. If he had not suffered any injuries, it would be easy for him to deceive and entrap Fortunato, because the poor man would have no reason to suspect any ulterior motive when Montresor lures him to his death. If Montresor has not been injured, then he is insane. But Poe could not have left the question of his sanity or insanity to hinge on a few words in the opening sentence of his story: "The thousand injuries of Fortunato I had borne as best I could, etc." If Poe wanted the reader to believe Montresor is insane, his creator would have given further evidence throughout the story. And if there is a plausible explanation for the "thousand injuries," that should refute the claim that Montresor is insane. To repeat: If Montresor really suffered a great many injuries from Fortunato, then he is sane.
It was a stroke of Poe's great literary genius that he began his story as if he were addressing a long-time friend in confidence. This eliminated the need for a great deal of exposition. For example, he doesn't have to explain where he lives, or who Fortunato was, or how long he had known him, or what their relationship was--or even what injuries he has suffered.
A clue to the "thousand injuries" can be found in the third paragraph of the story.
Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practice imposture upon the British and Austiran millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.
Now this says clearly that both men are experts who deal in one-of-a-kind items such as oil paintings, statues, antiques, and gourmet wines. Fortunato, as we learn, is rich and Montresor is hard up. Fortunato would have had countless opportunities to beat Montresor in business deals, both because he had more money and because, being Italian, he had more connections.
When Montresor tells Fortunato about his purchase of a cask of Amontillado, he knows his friendly enemy will see an opportunity to profit. They do not "buy largely" unless they are planning to sell. Fortunato goes with Montresor because he imagines a whole shipload of Amontillado newly arrived which he can buy up at a bargain price. Once again Fortunato would be beating his friend out of a deal, adding to the thousand injuries he had already committed. But this time Montresor--who is not so crazy--is one step ahead of him.
This would be a queer "revenge story" if there was nothing to revenge.