A protagonist is a character who is at the center of the conflict, driving the story. The antagonist is the person or force working against that character.
The protagonist of this story is Monstresor. First of all, he is the narrator - it is his story that he is telling. That doesn't always make the narrator the protagonist, but it often does. Secondly, he is the only character with any depth. Fortunato is a flat character, given little background and no opportunity to "grow" or expand. He does not drive the conflict - he does not even feel the conflict. For him, everything is fine, up to the moment he realizes he's being buried in a wall. The lack of detail about Fortunato - and about the supposed "insult" that he gave to Montresor - makes him too unimportant to be the protagonist.
It would seem that the antagonist is Fortunato, however. In Monstresor's mind, it is the force working against him, insulting him. Therefore, Monstresor must act out against Fortunato, as he does in murdering him. However, as said above, no detail is given about Fortunato's "insult", and readers are left with the impression that it either didn't happen or wasn't that serious. That makes Montresor his own antagonist - his resentment towards Fortunato, whatever the cause, is what is causing him to be conflicted and to act out in such a violent way.
Due to the fact that the story is written from the perspactive of Montressor he is the protagonist. Although it seems that he is not doing the morraly correct thing he is still the driving force behind he story. Subsequently, Fortunato is the Antagonist.
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