"The Cask of Amontillado" is Montressor's story. He is a man who tells the story of his revenge on his apparent nemesis, Fortunato. He does claim that Fortunato has committed a thousand injuries until he (Montressor) has had enough. We either have to believe him and take him literally at his word, or we have to test his reliability as a narrator and see those thousand injuries as hyperbole (exaggeration). I'll point out a few things, you watch for exaggerations, and you can make your own judgment from there.
Montressor claims a thousand injuries, yet when he is offered just one "insult," he is ready to seek revenge--to the death. Any exaggeration there?
When Montressor meets Fortunato for the first time at Carnival, he says this:
I was so pleased to see him, that I thought I should never have done wringing his hand.
This is a detail that doesn't really matter, yet Montressor feels the need to exaggerate even about a handshake.
Once Montressor has gotten the coughing Fortunato into the underground vaults, he says the following:
"Come," I said, with decision, we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill and I cannot be responsible.
This is an obvious exaggeration used to make continue luring his enemy into the catacombs of his villa.
If you've read the entire work, you know that Montressor is not above saying anything he needs to in order to get what he wants and convince you he's not really insane, just protecting his family honor (another exaggeration, of course).
Perhaps the most compelling argument that whatever Fortunato did was minimal or even inadvertent (accidental) and completely one-sided is the fact that Fortunato doesn't even really hesitate to go with Montressor. Surely an enemy worthy of murdering will have shown some real animosity, but that doesn't happen in this story.
Your assessment is valid. Follow this train of thought and you can make a strong and effective argument.
There are many parallels between the two narrators in the Poe short stories "The Cask of Amontillado" and "The Tell-Tale Heart." Both commit murders for unspecified grievances against their victims. Neither of the narrators can be called completely reliable. Certainly, Montressor comes across as the saner of the two men, though his ultimate crime is just as terrifying as in "TT-TH." Whether he tells the truth about the insults he has received from Fortunato is unknown; surely, the "thousand injuries" are an exaggeration, for Fortunato seems to be completely unaware of any anger that Montressor harbors. Montressor tells such a good story, and the reader is left wondering what great crime could have prompted him to take such actions against Fortunato. Since he was so willing to provide every other detail of his plan, why did he neglect to name the cause? You have every right to wonder about the reliability of Montressor's narration.