Because of the voice of the narrator--erratic, obsessed--the reader must wonder when Fortunato does not question the reason for Montesor's behavior if, indeed, Fortunato has really insulted Montesor. For, the opening lines are as ambiguous at the end of Poe's story as they are in the beginning. Has Fortunato really dealt Montesor a "thousand injuries" and has he "ventured upon insult" on Montesor, afterall?
Throughout the story does not Fortunato stand "bewildered" and "recoiled" from hints that Montesor ironically puts out for the connosieur? If Fortunato were the man to insult and injure so many times a man of Montesor's intrique, would he not have to be much more clever than he appears throughout the narrative even if he is inebriated?
Fortunato never apologizes for the "thousand injuries" or "insult" that he aimed at Montresor. That's because Montresor never directly tells Fortunato why he's being walled up and Fortunato is too surprised and also too insensitive to imagine why his friend would ask so harshly towards him. So Montresor is left with the "satisfaction" that Fortunato will no longer insult him, but only because Fortunato is dead, not because he was sorry for what he did to Montresor.
The reason Montresor never got true revenge was stated in his last paragraph. "For half of a century no mortal has disturbed them. In pace requiescrat!" He was hunted by what he had done. 50 years later he remembered the story as if it had just happened.