Montresor wants to get revenge on Fortunado for some unnamed insult.
Montresor does not specify how he was insulted, or what the final insult was that got him so mad.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
We do know that Montresor felt that he should be patient in getting his revenge, because revenge is not really revenge unless you escape punishment too.
Fortunado also suspected nothing. This is another reason we are led to believe that the injury or insult was a minor one, and mostly in Montresor’s head. After all, would you go underground to look at some wine if you thought you had insulted a man enough for him to want to kill you?
Poe intended to write a horror story about a man who commits a totally fiendish murder. The murderer, Montresor, had to have an exceptionally strong motive for doing what he did. Montresor claims to have been injured a thousand times. Poe is able to avoid having to give examples of what these injuries were. The tale is presented as a translation of an old manuscript detailing a crime committed at least fifty years earlier. Poe therefore poses as only the owner and translator of this old manuscript; he doesn't have to know anything about the thousand injuries--but the fact that Montresor was injured so many times (assuming we believe him) not only suggests why Montresor plans and executes such a terrible act revenge but also suggests that this Fortunato must have been a terrible man who deserved what he got. Poe's problems in writing the tale included keeping the reader somewhat sympathetic for a man who was capable of burying another man alive and leaving him to die of starvation. Poe deals with the important question of motivation in his opening sentence, because motivation is the most important consideration in any story, and Montresor's motivation for committing such a heinous murder needs to be made understandable and plausible.
This says everything that is necessary in a single sentence. Montresor's conduct from here to the end of the story will be driven by two forces, (1) the thousand injuries he has received, and (2) the fact that this proud man has made a vow to obtain revenge. The vow might be considered a more important aspect of his motivation than the thousand injuries, especially since Montresor says he had already borne them as best he could. Yet it must have been the "thousand injuries" rather than the "insult" that made Montresor vow revenge. The insult might have been trivial and might have only triggered the pent-up hatred caused by the thousand injuries.