Final answer:
Montresor seeks revenge on Fortunato for a final insult after enduring many injuries, aiming to punish Fortunato with impunity and exploiting his vanity in wine expertise.
Step-by-step explanation:
Montresor sought revenge on Fortunato because he had endured a "thousand injuries" and an ultimate insult from Fortunato. Montresor's pride and sense of wounded honor required him to plan a retribution that would go unpunished, as indicated in the text, "I must not only punish, but punish with impunity." Montresor's carefully calculated vengeance exploited Fortunato's pride in his wine connoisseurship, luring him into the catacombs of the Montresors with the promise of an exclusive Amontillado. Montresor's entire plan revolves around the knowledge that his retribution must not bring any harm to himself and that he must make Fortunato fully aware of the revenge being enacted upon him.