Montresor in Poe's "The Cask of Amontillado" is an effective villain and enemy to Fortunato for several reasons. The first reason is that he is unrelenting, and no hesitation or remorse interferes with his accomplishing his plans. Next, he is cool and calculating and patient, creating complex stratagems and waiting for the perfect moment to act, rather than being impetuous and rushed in his actions. Also, he researches his victim's habits carefully. Finally, he is good at dissimulating, pretending to be a friend to Fortunato so that his intended victim will not suspect his evil intentions.
A villain sharing many of the same characteristics is Iago in William Shakespeare's Othello. He is also a calculating villain who makes complex plans and feigns friendship with his intended victim to lure him to his fate.