Montresor never admits to a clear motive for killing Fortunato, speaking only in the vaguest terms about it. First, he says he has suffered a "thousand injuries" from Fortunato. We have to assume this is hyperbole, which is figurative language based on exaggeration. Since we know that Montresor is telling the story many decades after the fact, he must have been a young man when he murdered Fortunato. How could there have possibly been a "thousand" injuries? The fact that Montresor exaggerates suggests to us that he has an inflated or unrealistic idea of how he has been wronged.
However, Montresor brushes off the thousand injuries and states that, in fact, it is being insulted that determines him to pursue the path of revenge. An injury is actual damage or harm that a person endures. An insult is being spoken to or treated with disrespect. We can conclude from this that Montresor is a very proud person, who has little toleration for being disrespected. We know, too, that later on, Fortunato will speak with disbelief at the idea that Montresor might be a mason, thinking Montresor means he is a member of the freemasons. Fortunato says:
“You? Impossible! A mason?”
Montresor, however, means he is a literal mason who will wall Fortunato up to die.
It could be that Fortunato is simply a high-handed person who thinks he is better than Montresor, and Montresor, being mentally unstable, can't tolerate that. It could also be that he is walling up Fortunato due to some insult having to do with freemasonry.
The important point is that whatever Fortunato has done or whatever Montresor perceives him to have done, walling him up to endure a slow, horrible death in the catacombs can't possibly be commensurate with whatever "crime" he committed.