One of the intriguing aspects of "The Cask of Amontillado" is that we do not know, and cannot know, whether Montresor's relentless and horrific revenge is justified.
For example, Montresor establishes the reason for his hatred at the start of the story when he says
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge. You, who know so well the nature of my soul, will not suppose, however, that I gave utterance to a threat.
This disclosure tells us something very important about Montresor, specifically, that he is untrustworthy. Apparently, he has been the victim of a serious insult, but rather than address the problem openly--by challenging Fortunato to a duel, for example--he is disguising his feelings.
More important, however, is that Montresor never tells us what the nature of a "thousand injuries" is and how the "insult" was so qualitatively different that he had to revenge himself upon Fortunato. Because we are left to wonder throughout the entire story why Montresor is acting out the horrific revenge, we cannot but be suspicious of his motives and his sanity.
This is not to say that Fortunato is a sympathetic character--from his discussions with Montresor, he appears to be prideful, sarcastic, condescending. When Fortunato makes a sign of Freemasonry when he drinks, Montresor says that he doesn't recognize the sign, but that he is a Mason as well (alluding to his attention to wall Fortunato into the catacombs), but Fortunato, in a very condescending manner says, "'You? Impossible. A mason?'" Fortunato simply cannot believe that someone in Montresor's reduced circumstances could be a brother Mason.
Still, the fact that Fortunato is not a good person is no reason to kill him in such a horrible way as Montresor has planned. Because we never understand Montresor's motivation--what makes one insult unbearable--we are forced to conclude that Montresor, whatever Fortunato has done to him, is unbalanced and unreliable.
The question of whether Montresor's revenge is justifiable, then, is not answerable given the information we have. It is even difficult for us, despite the horrible crime Montresor commits, to render a moral judgment against Montresor: if we knew what "insult" had had suffered, perhaps we could conclude that his revenge is justified or unjustified, a conclusion based on our individual beliefs. But, in the end, we are left as mystified as Fortunato is when the last brick goes into place.