In Edgar Allan Poe's "The Cask of Amontillado," Montressor is using extreme exaggeration (hyperbole, verbal irony) when he says:
THE thousand injuries of Fortunato I had borne as I best could ; but when he ventured upon insult, I vowed revenge.
Montressor provides neither the injuries nor the insult in the story, which makes us believe that it is the other way around (Montressor is delivering all injuries and insults). Indeed, Montressor is more than a bit paranoid--to the point of mania. His overdeveloped sensitivities (regarding his family code of revenge: "I must not only punish, but punish with impunity") have lead him to imagine these violations, and his elaborate plan of revenge reveals signs of his own mental illness. It seems that family reputation (which prides in revenge) has driven him to look for signs of insult that--to an average person--are unintended or non-existent.
At worst, Fortunato is guilty of drunkenness, which is a self-induced injury that should not offend Montressor. In fact, Fortunato's drunkenness plays into Montressor's plan of revenge. Perhaps Fortunato is materialistic in his connoisseurship of wine: he must have it at all costs. This too plays into the plan, but it is not an overt threat.
Nothing from Fortunato's statements in the story lead us to believe even a hint of malice by him toward the narrator. Sure, he forgets Montressor's coat of arms and family motto. Big deal. Sure, he insults Lechesi, calling him an "ignoramus," but this should not offend our narrator. In the end, Fortunato remains completely oblivious of Montressor's intentions and his fate.
No, the thousand injuries and insults are indeed delivered by Montressor, not Fortunato, which makes this story a classic tale of motiveless revenge and extreme paranoia.
Many people have questioned Montresor’s assertion that he had received a thousand injuries at the hands of Fortunato and base their doubts on the fact that he apparently fails to describe even one injury. But it seems impossible to believe that a literary genius and perfectionist like Poe would ignore such an important matter and thereby create confusion about the honesty or sanity of his narrator Montresor. I believe that an explanation of the “thousand injuries” is to be understood by a close reading, with particular attention to the third paragraph.
He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian MILLIONAIRES. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.
This paragraph does many things. It distances Montresor from Italians and helps to identify him as an outsider. It strongly suggests that both he and Fortunato deal in expensive merchandise such as painting and gemmary (jewelry) and “old wines.” Both are aristocrats, but they have to earn money in the decaying city of Venice where many formerly wealthy families are selling off their possessions in order to stay alive. Montresor and Fortunato are gentlemen “dealers” or “brokers” but not merchants.
Fortunato is rich, but Montresor is poor. If he has been insulted by Fortunato a thousand times, why does he continue to associate with him. No doubt he is dependent on Fortunato in business transactions. He might borrow money at interest to purchase an oil painting for resale, or he might have to take Fortunato into a temporary partnership on a transaction because he could not afford to handle it with his own limited resources. Montresor in some cases might only receive a finder’s fee on a lucrative deal he originated himself. Fortunato would have ample opportunities to take advantage of him. For instance, if Montresor proposed a fifty-fifty split of the profits on a sale of a statue or oil painting, Fortunato might hold out for sixty or seventy percent, and Montresor would have no choice but to agree. These are the types of injuries Fortunato has suffered, and this is the explanation of why he continues to associate with the man and to call him his “good friend.”
Monresor knows his man. He knows that Fortunato is not anxious to taste his nonexistent Amontillado out of friendship or to show off his connoisseurship. Nor is he strongly tempted to sip a glass of wine in the dank, dark underground vault full of human bones. He is planning to taste the wine, shake his head and judge it to be inferior sherry--then rush off to find the newly arrived Spanish ship and buy up the entire cargo at a bargain price before Luchesi ever hears about it. Montresor undoubtedly would have purchased more than one cask if he had been sure it was genuine, but he would find it has been all sold off. And his good friend will call his dirty trick "an excellent jest." Montresor is only double-crossing the double-crosser.