Montresor's failure to tell us what insult he has suffered at the hands of Fortunato puts us on our guard from the first sentence of the story. Although Montresor seems to be able to bear Fortunato's injuries, he can't ignore the insult, whatever that is. As reasonable people, we have to assume, at least at the beginning of the story, that the insult must have been horrific and therefore unbearable. The fact that Montresor doesn't disclose the nature of the insult, though, should make us a bit cautious about Montresor as narrator.
Because the story is told through Montresor's eyes, we have to rely on his truth, his perception, his understanding, and his judgment as we move through the story. As the story unfolds, and we see more of Montresor's devious and vengeful nature--confirmed by his family's coat of arms--and we begin to understand that Montresor is a seriously flawed person and an unreliable narrator.
By the end of the story, especially when we see what Montresor does to Fortunato, we cannot help but be very uneasy about Montresor's justification. His failure to disclose the insult is Poe's way of warning us that we are in the hands of a madman.