In "The Cask of Amontillado," by Edgar Allan Poe, Montresor's motivation for killing Fortunato is simple: he was insulted. In the opening line of the story, Montresor (narrator) explains that he suffered through many hurts imposed by Fortunato, but would not put up with an insult: "The thousand injuries of Fortunato I had borned as I best could, but when he ventured upon insult I vowed revenge," (1).
It is unclear what the insult was regarding, but one might guess it had something to do with familial protection, as family seems important to Montresor, (illustrated in his discussion of his family's coat of arms); or it may have to do with wine connoisseurship as both men seem to pride themselves on this skill; "I was skillful in the Italian vintages myself, and bought largely whenever I could," (2).
Interestingly, the family motto of Montresor is "Nemo me impune lacessit," meaning "No one attacks me with impunity." This implies that members of the Montresor family do not allow anyone to hurt them and then get away with it. However, Montresor's plan to wall Fortunato up inside the catacombs without anyone ever knowing about it is an illustration of exactly that. In fact, Montresor is never punished for his deed because he's never found out: "For the half of a century no mortal has disturbed [his bones]," (6).