There are only two characters in Edgar Allan Poe's classic short story "The Cask of Amontillado"--Montressor and Fortunato. But which one is the protagonist? By normal definition, the protagonist is the main character of the story, in many cases considered even heroic, but nearly always it is one that receives empathy or emotional support from the audience or reader. Technically, Montressor would be the protagonist, although most readers would not consider him heroic or a character worthy of their moral approval. This would leave Fortunato, by default, as the antagonist--the adversarial character in main opposition to the protagonist. However, most readers probably sympathize with Fortunato's fate--a response usually reserved for the protagonist. The ambiguity of the two characters is just another example of the brilliance of both the story and its writer.
The protagonist is Montressor and the antagonist is Fortunato.