So sometimes you see somebody in a movie or read about somebody in a story who gets killed and you are like - yeah, he deserved that. Fortunato, is not one of those people. Poe completely mocks the poor guy, and Montresor totally just kills him and you are sitting there wondering - WTF?
What did Fortunato do to piss him off so bad? The guy seems nice enough. He could have said "piss off, its carnival, get Luchesi to help your ignorant ass." But instead he says, hey, I'll follow you down into your cold, damp, stinky catacombs to sort out your stupid buying practices. A nice guy! And what does he get in return? Death. Poe - you are messed up!