First paragraph: why does the narrator, Montresor, never reveal exactly what the "insult" was that prompted him to seek revenge against Fortunato?
Take a careful look at the story's first paragraph: why does the narrator, Montresor, never reveal exactly what the "insult" was the prompted him to seek revenge against Fortunato?
You are right. Montresor doesn't specify the insult he received from Fortunato. But he doesn't mention any of the thousand injuries either. Some critics have speculated that Montresor is insane and that he never received any injuries or insults from Fortunato. I personally believe that Poe wanted to make his story as short as possible, in accordance with his rationale that there should be no unnecessary word in a short story; and if he got into describing a lot of injuries it would add many words of exposition. Critics would still be free to doubt whether any of the injuries he cited were real or imaginary. If we can't take Montresor's word that he received a thousand injuries and at least one insult, then how can we take his word about anything in his story? Everything else he says seems sane enough to me. Isn't that a good way to judge his sanity?
I think Poe wanted the reader to gather some idea about the relationship between Fortunato and Montresor during the course of the story. Fortunato is not interested in the Amontillado because he wants to drink it or because he wants to show off his connoisseurship. Amontillado is a sipping sherry. If he drank a lot of it he would get sick. What interests him is that Montresor got a bargain on a whole cask of it. From what Montresor says early in his narrative, it appears that both he and Fortunato deal in art, jewelry, and probably wine--any expensive items that would appeal to millionaires. But Montresor is obviosly poor. If a ship from Spain brought in a cargo of Amontillado, Fortunato might be able to buy many casks at an even greater bargain price. Both men refer to the cask as a "pipe." A pipe of wine is 126 gallons. Fortunato, who is rich and well connected, has probably been able to beat Montresor out of many bargains in art works, jewelry, ceramics, tapestries, antiques, and wines over the years, and I presume these constitute the bulk of the "thousand injuries" Montresor has received. Why? Because how else could any man have inflicted so many injuries on another man if they weren't competitors? What is the difference between an injury and an insult?
Evidently the insult wouldn't have impelled Montresor to seek such terrible revenge if he hadn't suffered a thousand injuries before. The "insult," whatever it was, was the straw that broke the camel's back. Montresor's motivation is of the utmost importance, but does it matter whether or not it is justified?