Montresor is the narrator of Poe's short story The Cask of Amontillado. The reader does not learn that key bit of information until near the end of the story when Fortunato screams out and begs for his life.
At the very beginning of the story, Montresor announces to his reader why Fortunato must die. Montresor kills Fortunato out of revenge. More specifically, the reason for his revenge is that Fortunato insulted Montresor in some way.
THE thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge.
Montresor never explains what those injuries were or what the insult was. It is up to the reader to hypothesize a bunch of possible reasons. Fortunato is pompous and annoying for sure, but Montresor isn't exactly pure as the driven snow. If he is killing somebody over a single insult, he is definitely a tense individual.