The very first line of the story is an example of hyperbole, or overstatement. The narrator, Montresor, says, "The thousand injuries of Fortunato I had borne as best I could; but when he ventured upon insult, I vowed revenge." It is unlikely that Fortunato had actually injured Montresor a thousand times, and thus we can deduce that Montresor, the narrator, is exaggerating in order to make a point.
It seems that Montresor is now an old man, confessing his sins on his deathbed, it is probable that he is trying to justify his behavior -- and the murder of Fortunato -- and really impress upon his listener how deserved such an action was. Since the purpose of hyperbole is always to emphasize the truth by exaggerating the truth, he also seems to want to convey just how injured he truly felt. Even if Fortunato had not hurt him a thousand times, it certainly felt that way to Montresor, and thus he felt compelled to act the way he did.