It is possible that Poe intended to create the impression that the narrator was speaking aloud to a confessor, but it seems more likely that he wanted to create the illusion that his story was a translation of a personal letter that had somehow come into his possession. Otherwise, we would have to imagine that the narrator was speaking in either Italian or French and that somehow we were able to understand the words as if we could hear them in English. This would detract from the perfection of Poe's story. It would be a further stretch to assume that the narrator also happened to know English and for some strange reason chose to make his confession in that language. Furthermore, if the narrator were speaking aloud, perhaps on his death bed, he would be unlikely to remember all the fine details he includes in his account of an incident that occurred fifty years before. More likely, he was writing a confidential letter and took plenty of time to remember the events he was describing. He never uses phrases such as, "I seem to recall," "If I remember correctly," "He said something like . . ."--Poe might have used such phrases if he wanted to create the illusion that the narrator was speaking aloud to a second party. The best explanation of the fictional format is that Poe found an old letter written in Italian or French and translated it into English for publication in an American magazine. It is possible that the letter was never even mailed but was found among the narrator's papers after his death. Many confidential letters are written, often while the writer is intoxicated, and never mailed.
The narrator does not seem to be speaking to anyone in particular. Some readers might suggest that the conclusion (paragraph 89) indicates that the narrator is close to death and is making a confession to rid himself of guilt. If that were the case one would expect an interaction between the narrator and the confessor, much in the style of a dramatic monologue. There is nothing like that, and therefore Poe is able to concentrate on the luridness of the tale. If Montresor explained the insult, we as readers would know little more than we do, but might become diverted in trying to justify Fortunato and therefore water down the impact of Poe’s tale.