Edgar Allan Poe's short story masterpiece, "The Cask of Amontillado," tells the story of a man's desire to gain the ultimate revenge upon a man who had borne him a "thousand injuries"--though the acts of unkindness are never revealed by the murderous narrator, Montresor. Montresor has decided that his acquaintance, Fortunato, must die for the unnamed wrongs and insults that have been made against him. Montresor plans his revenge carefully, making certain that two things will be accomplished: He will not be caught--there will be no form of retribution; and Fortunato must recognize, before dying, that Montresor is his killer. Montresor succeeds on both counts.
Luring Fortunato into the Montresor family catacombs--which doubles as a wine cellar--on the pretense of sampling a rare bottle of Amontillado, Montresor leads him into the far recesses of the crypt. When Fortunato, who is already drunk, is caught unaware, Montresor chains him to a wall and proceeds to seal up the remaining side with mortar. There, Montresor leaves Fortunato to die, where
For the half of a century no mortal has disturbed them.
Why Montresor waits for 50 years before revealing his secret is unknown. Perhaps he is on his deathbed, and he wishes to ease his conscience in the presence of a priest; or maybe he is passing on his tale to a surviving relative or friend. In any case, Fortunato's murder will not be discovered during Montresor's lifetime, and Montresor can revel in the fact that he committed the perfect crime against his greatest enemy.
Montresor is not a real person but a character invented by Edgar Allan Poe. Montresor waits for fifty years to tell his story because Poe wanted him to wait for fifty years. Poe was writing a perfect-crime story in which the criminal actually gets away with murder. This was probably unprecedented at the time. Editors didn't feel morally justified in publishing a murder story in which the murderer doesn't get caught. (It used to be the same with Hollywood movies during the reign of the Hays Office. Murderers had to get caught. Billy Wilder'sDouble Indemnityis a good example.)
So Poe placed his story in the distant past and in a distant country where the criminal at least would not have broken any American laws. I believe that if Poe had written the same story with American characters in an American setting and in his present-day time period, he would never have gotten it published--especially in a magazine for ladies! But Poe was an editor himself, and he would never have written his story in that context. He wrote other perfect-crime stories in which the murderer was caught because he overlooked something, notably "The Tell-Tale Heart" and "The Black Cat."
Poe shows his exceptional genius in many ways, including having a foreign narrator tell about a crime he committed fifty years ago.
Montresor is probably on his deathbed--in fact, the person to whom he is speaking is someone about whom he says "You who so well know the nature of my soul", so it's probably a priest. It is with this confession that we learn the most about Montresor--he has been haunted by his need for "retribution", which goes against what he had said about retribution only being retribution when one gets away with it. Instead of "getting away with the perfect crime", Montresor has been haunted by it, and this confession is the only thing he can do to clear his conscience.