What kind of man is Montresor?
Montresor is an extremely proud man. It is the fact that he feels so injured and insulted by Fortunato that causes him to seek the most violent and permanent revenge possible: he wants to kill his nemesis. He is proud of his family motto, "Nemo me impune lacessit," which translates to You will not harm me with impunity. Montresor feels wronged, and so he cannot maintain his family honor and pride unless he lashes back at the one who he feels has harmed him.
Montresor is also a very clever man. He understands people: he knows that if he tells his servants not to leave and that he will be out all night, they will undoubtedly leave immediately. He also knows Fortunato well enough to know that this man's pride can be used against him: Fortunato will insist on seeing the rare and expensive wine in order to gloat over the fact that Montresor was swindled; he will insist despite the danger to his health and despite the fact that another wine connoisseur is available. Further, Montresor is careful to think ahead and dress is such a disguise that he cannot be identified later as having been seen with Fortunato. He seems to think of everything (except the guilt he may feel later on...).
Montresor is vengeful, obsessive, methodical, deceitful, manipulative, and merciless. Repaying Fortunato for an unnamed "insult" that is more important than the "thousand injuries" he has suffered at his hands becomes more than just an idle fantasy. It obsesses him to a point that he devises a careful, cold-blooded, step-by-step plan to murder his enemy, leaving nothing to chance. He deceives Fortunato by playing on his vanity and trust. He picks a time, Mardi Gras, when Fortunato will be inebriated and vulnerable and lures him into the damp, isolated catacombs. Fortunato, his judgment impaired by drink, clearly does not suspect that Montresor is his enemy. In conclusion, Montresor shows no mercy as he chains up his enemy leaving him to die slowly and alone in a dark place.