What is it about Montresor that makes him an especially effective enemy to Fortunato?
Montresor is an especially effective enemy of Fortunato for a few reasons, not the least of which are his intense pride and ability to manipulate. As he says early on, "I must not only punish but punish with impunity." He feels the need not only to avenge the wrongs done and insults given to him by Fortunato, but also to exact his revenge without having to endure some punishment for it. His family motto translates to "You will not harm me with impunity"—in other words, no one can harm a Montresor and get away with it. His personal and familial pride make him especially relentless and ruthless when it comes to Fortunato.
Further, Montresor clearly put a great deal of thought into his revenge, and he manipulates his staff and the public, as well as Fortunato, to achieve it. He tells his servants he will be gone all night and that they must remain at home, knowing they will all go to the festival as soon as he leaves (and will claim they were home all night to avoid trouble). He makes sure his costume covers his face so he will not be seen with Fortunato in public, and he concocts an elaborate story about a pipe of Amontillado, knowing Fortunato's pride will compel him to risk his own health in order to prove Montresor has been had. He is so thoughtful and manipulative, which makes him a perfect enemy for the relatively simple and straightforward Fortunato.
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