Montresor says that his motive for killing Fortunato was that Fortunato had insulted him.
We actually do not know what Montresor thinks Fortunato did, but we can assume it was nothing significant because he is not specific, and because Fortunato does not seem to be aware that there was an injustice done at all.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that gave utterance to a threat.
Fortunato goes with Montresor down into the catacombs at night. No one would do that if he felt that a person was out to get him for revenge. You just do not go underground with people you have mortally insulted!
Yet Fortunato has no idea that he ever insulted Montresor or that Montresor is harboring a murderous rage because of it.
It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend. He accosted me with excessive warmth, for he had been drinking much.
Would you really show someone excessive warmth if you thought they were holding a grudge against you? No, you would more likely be very cautious. Montresor gets away with murder because Fortunato has no idea that he is even angry.
The hyperbole in saying that Fortunato committed a “thousand injuries” and the fact that Fortunato is not suspicious adds up to the idea that Fortunato did not really do anything. Montresor imagined it. There was probably some minor slight that no one else would have noticed, which Montresor blew out of proportion.
Montresor is clearly a madman. Madmen do not make very good friends. He is having some kind of delusion about Fortunato, and because of that Fortunato has to die.