This is such an interesting question because in the very first sentence, the narrator tells us, "The thousand injuries of Fortunato I had borne as best I could, but when he ventured upon insult I vowed revenge."
The narrator is trying to convince us from the very beginning that Fortunato has wronged him so unjustly that Fortunato deserves death. That is a pretty strong position to take, so where is the support for these "thousand injuries"?
Interestingly, the narrator doesn't really develop those details. And that is part of the problem, because this narrator has many qualities that lead us to believe he is unreliable and that maybe the details he is providing aren't exactly factual. (Poe presents similar narrators in "The Pit and the Pendulum" and "The Tell-Tale Heart.")
In the second paragraph, the narrator tells us that "it must be understood that neither by word or deed had I given Fortunato cause to doubt my good will." In effect, the narrator is therefore bragging about how well he was able to deceive Fortunato into trusting him on the walk that led to Fortunato's death. If he so effectively deceived his victim, is he equally talented at deceiving the reader?
And it is also important that in the final lines, the narrator lets us know that it has been "half a century" since this murder took place. So how clear are his memories of these "thousand injuries," anyway?
It is possible that Fortunato inflicted no injuries on our unreliable narrator at all. Surely those would have been worth recounting as justification for the murder that transpired, so the absence of these key details really builds a case that Fortunato may not have been to blame for his untimely end.