In the first line of the story, the narrator vows revenge on Fortunato because he claims he had insulted him in the past.
The thousand injuries of Fortunato I had bourne as I best could, but when he ventured upon insult I vowed revenge.
However, he doesn't really say how Fortunato had insulted him, and he even refers to him throughout as "my friend." Fortunato seems to be completely unaware of any bad blood between them. The only clues Montresor gives as possible offenses are his family emblem and the suggestion of a possible disbarment from the masons.
Montresor's family motto, for example, underneath a picture of a foot crushing a serpent, is "Nemo me impune lacessit," meaning, "no one attacks me with impunity." This could suggest that the attack was not on Montresor personally but on someone in his family, or even on his family as a whole. After all, he doesn't seem that comfortable in confronting Fortunato about what happened. It is almost as if he didn't experience it firsthand.
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