The most puzzling and intriguing lines in "The Cask of Amontillado" are undoubtedly,
The thousand injuries of Fortunato I had borne as best I could; but when he ventured upon insult, I vowed revenge.
At this point, we reasonably expect Montresor to identify the nature of the "insult," particularly when it becomes clear that the insult, whatever it is, has driven Montresor to take such horrific revenge on Fortunato. Fortunately, however, for many generations of readers and literary critics, Montresor leaves everyone mystified about the nature of the insult, which in turn casts doubt upon Montresor's reliability both as the narrator and as a person. From a practical standpoint, if readers cannot trust Montresor-as-narrator, they can never be certain whether he is recounting events or personalities as they are or as he perceives them. For example, our perception of Fortunato may not be accurate because we see him through the eyes of an unreliable (that is, obsessed) narrator, one of Poe's favorite character types.
One of the most important aspects of "The Cask of Amontillado," then, is not what we know about Montresor and his revenge, but what we do not know. His failure to identify the insult, among other things, leads readers to debate endlessly about the cause of Montresor's revenge and, perhaps more important, gives the narrative an air of unresolvable mystery. Without knowing the cause of Montresor's hatred, it becomes impossible for us to decide whether Montresor's revenge falls within justifiable behavior--assuming, of course, that anything justifies murder.