Montresor, Poe's unreliable and hyperbolic narrator, claims that he seeks revenge after Fortunato has added insult to injury.
In the exposition of Poe's Gothic tale, Montresor claims that he has endured "the thousand injuries" that Fortunato has committed against him; however, when his enemy has "ventured upon insult," he states that he can bear no more, and must be avenged. Having decided upon revenge, Montresor commences his intricate plan to approach Fortunato during the Carnival season when Fortunato's disappearance should not soon be noticed. Also, Fortunato, who should be at least somewhat inebriated from celebrating, will be more susceptible to Montresor's luring him into the catacombs on the pretext of tasting the Amontillado.
Montresor's plan is effective as he succeeds in tempting his enemy Fortunato into the damp "vaults." Further, Montresor exploits Fortunato's desire to outdo his rival Luchesi by tasting the Amontillado. Montresor also feigns concern for Fortunato's health because of the dampness of the cavern walls and repeatedly suggests that they turn back. But Fortunato, who will not be outdone later by Luchesi or anyone else, insists that they keep going forward. As Montresor knows, Fortunato is a rapacious man who wishes to taste the Amontillado and judge it before his foe Luchesi has any chance to do so.
Montresor decides to seek revenge against Fortuanato because he believes that Fortunato has insulted him. The story says "the thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge." We are not told the specifics of this insult. The story leaves the reader to wonder what the insult was and if it ever actually occurred. The story also describes Montressor's family coat of arms and moto. The coat of arms depicts a large foot crushing a snake that has bitten the heal of the foot. His family motto states "no one attacks me with impunity." This tells the reader something of Montressor's character. He feels that he must punish any offense. Montressor does not like Fortunato and feels he has put up with him long enough. Finally, Fortuanto insults Montressor in some fashion and Montressor's anger boils over. Once again, we do not know if this offense ever really occurred.