The motive for revenge in the short story, “The Cask of Amontillado” is simple insanity. The reader who is looking for a specific cause will find there is really nothing specific indicated beyond Montresor’s madness. He says in the beginning, “The thousand injuries of Fortunato I had born as best I could.” This is the only reference. However, there are plenty of relevant clues to his madness: “It was about dusk, one evening during the supreme madness of the carnival season;” the unmerciful way in which he torments Fortunato; pure evil of his plan; the way he mocks Fortunato’s last moments; and the relish with which he tells the tale after 50 years.
Enotes has some great resource material.