You don’t need us to tell you that Montresor is the most obvious antagonist in “The Cask.” He wants to wall a guy up in a crypt over an insult. Not just wall a guy up, but torture him before he dies.
Part of his power as a villain is in his omission of the details of Fortunato’s alleged insult against him – and of the “thousand injuries” that came before the insult. We want to believe that nothing Fortunato did could possibly warrant Montresor’s treatment of him, but, without any other information, a shadow of a doubt remains: maybe Fortunato deserved it. What if Fortunato killed Montresor’s family or something? Would that make a difference? This question is part of what makes this story more than just a spooky tale.
From Montresor’s point of view, Fortunato is the antagonist. You know why – the mysterious injuries and insults. And don’t you get the feeling Fortunato would do anything for the Amontillado? Maybe he would even kill Montresor for it. Remember, like Montresor, he’s trying to be sneaky. He keeps saying that Montresor couldn’t possibly have real Amontillado. He’s just going along to help prove it, drinking Montresor’s wine, pretending he’s doing Montresor a big favor by testing the Amontillado for him.
Also remember, it’s not just a bottle of the stuff, but a whole barrel. Fortunato seems really greedy about Amontillado – he’s going to want to find a way to get it away from Montresor. Of course, he wouldn’t be down there in the first place if Montresor hadn’t wanted to kill him. But that’s beside the point. The real point is that there’s something creepy and antagonistic about Fortunato. This amplifies the mystery and the suspense of the story.