Readers no doubt learn that Montresor is truly a vicious man, whose concept of revenge is to punish entirely (i.e., to kill), and that the vengeance must be carried out with no harm to the revenger. Poe’s concept of revenge is therefore unlike the revenge tradition inherited by Shakespeare, for in the Elizabethan tradition of literary revenge, the avenger always dies, as most watchers of any Shakespeare tragedy will testify to. Montresor is interested in living and therefore does his work secretly, away from all prying eyes. He is even able to keep the cement wet enough for him to return and finish what he started.
In The Cask of Amontillado, Montresor can be perceived as both the antagonist, as well as the protagonist for various reasons. Montresor can be seen as the antagonist because, quite simply, he murdered a man. However, he can also be seen as the protagonist because to have killed Fortunato, he would have needed a motive, which could be supported by the first line ("the thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.") Overall, it probably makes more sense to give Montresor the title of the antagonist because, although he did have an incentive, his reason was not strong enough for full on annihilation.