Montresor says that he had borne Fortunato's thousand injuries but vowed revenge when Fortunato "ventured upon insult.l" This seems to suggest that the specific insult or incident was not what was important but rather the fact that Fortunato's attitude and behavior were becoming more insolent and intolerable. Montresor seems to be implying that he felt that Fortunato was testing him to see if he would put up with verbal abuse. If so, Montresor could expect to be insulted again and again, and he could expect the insults to be increasingly hurtful. Perhaps the initial insult was relatively mild and not even premeditated.
One certainly wonders why a man with Montresor's proud and prickly nature would continue to put up even with "injuries." The only logical explanation is that Montresor must have been dependent on Fortunato for money. The third paragraph in the story strongly suggests that both men are dealers in expensive wares. Montresor mentions painting, gemmary, and old wines. He also mentions British and Austrian millionaires. Later in the story Montresor states that he is poor and Fortunato is rich. It seems likely that Montresor makes part of his income by such means as collecting finder's fees by referring buyers and sellers to Fortunato. For example, an impoverished Venetian aristocrat might want to sell an oil painting in order to meet his family's living expenses. Fortunato might be able to buy the painting for resale where Montresor could not. Fortunato also seems to be better connected socially than Montresor. If a seller wanted someone to act as a broker in liquidating an valuable heirloom, such as an antique or a piece of jewelry (gemmary), he might be more likely to trust Fortunato than Montresor.
Poe himself was super-sensitive about his financial and social position. He had once been treated like a foster son by a wealthy man and had fallen out of that man's favor. Afterwards, he was always living on the brink of financial disaster and subject to social snubs and cruel gossip. Poe was especially vulnerable because he had a drinking problem and had married his cousin Virginia when she was only thirteen years old.
It seems reasonable that if Fortunato had really offended and insulted Montresor, he would have been wary of Montresor's motives for leading him farther and farther into the damp depths of the catacombs. Surely, too, he would have broached the subject when Montresor fetters him and begins to wall him in, even if he were inebriated. The real horror of this story lies inside the mind of Montresor and his capabilities.
We do not know what the insult was, but I think it was real (unlike the old man's evil eye in "A Tell-tale Heart" for example). I do not think it was a serious slight. More than likely most of us would think it minor. I don't think Fortunado would have gone into the crypt with him if he had thought he insulted Montresor and feared reprisal.
I know some literary critics have gone so far as to speculate not only that Fortunato actually did offend Montressor, but they claim they know exactly what the insult was--Fortunato had an affair with Montressor's wife. (And some even speculate further that Montressor killed his wife and buried her in the very catacombs in which Fortunato dies.) I see nothing in the text which would lead to that conclusion. In fact, every time I read it, I look and listen again to see if Fortunato gives the slightest indication that he's got any clue or any enmity at toward Montressor. I just don't see it. Somebody's imagining things, and I'm guessing it's the critics and the narrator--Montressor.
Do you think the insult to Montresor was all in his mind?
The nature of the insult Fortunato offered Montresor is never clear. Do you think it really happened or it occurred only in Montresor's mind?
This is an interesting question, especially considering who wrote the story. I've always wondered if the raven actually said "Nevermore." Poe's narrators really make us stop and think about their stability and judgment. It is hard to put much faith in a narrator who hears a human heart beating under the floorboards where it had been hidden after being ripped from its owner's chest . . . and the examples go on.
Whether Montresor had been truly wronged or not is really beside Poe's point. His stories generally do not emphasize theme. ("The Masque of the Red Death" is an exception, I think.) Poe's theory of the short story stressed that the story should be read in one sitting and that everything in a story should contribute to one emotional response in the reader. The story was to achieve one primary effect. In Poe's work, of course, the desired effect was horror.
Whether Fortunato's insults were real or imagined, Montresor certainly believed they had been inflicted. The depth of his outrage leads him to the most diabolical revenge and Fortunato's growing horror as he slowly realizes what is happening to him.
It is very possible that the insult was all in his mind. He gives several indications that he is mentally unstable...the maniacal laughter and the taunting that he delivers both on the way to the wine cellar and during the bricking of the wall with Fortunato inside it.
How many times has someone "insulted" you and you didn't tell him or her? Montressor says Fortunato insulted him, but it is very possible that Fortuanato was completely unaware of the offense.
That's a good question, but how to answer?
This is pure speculation, but Montresor did speak of "a thousand injuries." Surely this is an exaggeration by itself, but there must have been some offense committed. Montresor also speaks of Fortunato's social position and prestige ("You are a man to be missed..."), something he evidently lacks. Perhaps he refers to something Fortunato did much earlier, or eventually one of his ancestors. Remember the coat-of-arms of the snake being crushed by the golden heel, a clear emblem of vengeance. This could have referred an age-old family feud never really settled until Montresor takes things in hand...
Whether the crime was real or imagined, done by Fortunato himself or committed by a forefather, we cannot know. What we do know is that Montresor held Fortunato personally responsible for his family's undoing, and that was reason enough for him lure Fortunato into the crypt and kill him there.
Bear in mind that the ambiguity between reality and imagination in Poe's works is intentional and is one of the trademarks of his genius.