We do not know that the concept of revenge is only in the mind of Montresor. Some readers believe he has no valid reason for wanting revenge, while others believe he has suffered a thousand injuries at the hands of Fortunato. We are pretty much forced to rely on Montresor because he is narrating events that only he knows about. The story of the Amontillado is not "real" in more than one sense. First, it was created by Edgar Allan Poe. It never really happened. The characters never really existed except in Poe's imagination. Second, one of those characters invented the cask of Amontillado in order to lure the other character down into his catacombs. There was no cask of Amontillado. Does it make any difference whether Montresor has a valid reason for wanting revenge or whether he only imagines that Fortunato has injured him? Montresor wants to murder Fortunato, and he succeeds in doing so. That is what the story is about. The only reason there could be for suspecting that Montresor does not have a valid excuse for committing the murder would be that Poe fails to give examples of the "thousand injuries" Montresor thinks he has suffered. Poe made a decision not to elaborate on those injuries, but he probably wanted his readers to believe they were real injuries and not imaginary. You seem to be suggesting that Montresor made up the entire story about the murder and that none of it really happened. This gets pretty complicated, since it is quite true that none of it really happened. Did Poe make up a story that never happened about a man who made up a story that never happened? This is a little too post-modernistic for the time.