In "The Cask of Amontillado, the character Fortunato plays the role of the antagonist, against the protagonist, narrator Montresor. An antagonist is a character or group that is against the antagonist or hero of the story. Since Montresor is the narrator, and the one claiming to be abused by Fortunato in the past, Fortunato is the antagonist.
Although Fortunato is the character murdered in the story, it is because of past wrongs that Montresor feels justified in entombing Fortunato alive. Montresor killing Fortunato does not automatically make him the antagonist, as part of the role of an antagonist is to create conflict or trouble for the protagonist.