Characterize Montresor. What kind of person is he?
Montresor is an extremely proud man; he is also very clever and manipulative. At the beginning of the story, he exaggerates the number of injuries he had sustained at the hands of Fortunato, as if to justify his murder. He says,
The thousand injuries of Fortunato I had borne as best I could, but when he ventured upon insult I vowed revenge [....]. At length, I would be avenged; this was a point definitely settled [...]. I must not only punish but punish with impunity. A wrong is redressed when retribution overtakes its redresser.
His pride will not allow him to labor any more under the insults with which Fortunato has apparently assaulted him. He must seek revenge, and it must be done in such a way that he can never be punished for it (or else it doesn't really qualify as revenge because he'd be harming himself in the process). Montresor feels that he must live up to his family motto: "No one harms me unpunished." He clearly feels a great deal of family pride, as he tells Fortunato, "'The Montresors [...] were a great and numerous family.'" Because Montresor speaks in the past tense, here, we might assume that his family is no longer as great or numerous as it once was, and this might be another reason why he feels so strongly about honoring the family by upholding their motto.
Further, he thinks he knows just how to move forward with his plan to exact his revenge "with impunity," and he very nearly does achieve it. He is quite cunning while preparing a trap to catch Fortunato, ironically, with Fortunato's own pride. Montresor says,
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.
He wants to be sure that his auditor understands how craftily he proceeded with his plan. Montresor tells us that Fortunato has one weak point, and though he never names it directly, we can assume that it is Fortunato's own pride, especially in his talent and taste as a wine connoisseur; Montresor says that "in the manner of old wines, [Fortunato] was sincere." Montresor rather brilliantly exploits this one weakness in order to exact his revenge. He engages Fortunato's pride by telling him that he bought a type of rare wine and that he was looking for another local expert to help him confirm the wine's identity, so to speak. Fortunato cannot turn down an opportunity to showcase his talent (or rub Montresor's nose in his likely error).
What Montresor doesn't count on, however, is his own guilt. He planned for everything except the way his own conscience might punish him. It seems that, even though he was never formally punished for Fortunato's murder, his guilt has lingered for some half a century and this, perhaps, has actually been his punishment. The fact that Montresor seems to be an old man now, on his deathbed, confessing the sins which still weigh heavily on his conscience, tells us that the murder has stayed with him. He is telling this story to one who he says, "so well know[s] the nature of [his] soul," and the final Latin line that translates to "rest in peace," seems to support this reading as well. Further, when he describes his feelings after he'd walled Fortunato in, he says, "My heart grew sick; it was the dampness of the catacombs that made it so." This, again, sounds like someone trying to convince himself not to feel guilty, that his actions were warranted, even justified, and there is really no reason to convince ourselves that we shouldn't feel guilt if we already don't.
Thus, Montresor is quite proud, and very intelligent...just not quite as intelligent as he believes himself to be because he failed to account for the way a guilty conscience could punish him for the remainder of his life.