An unfortuante person who has suffered injustice or cruel treatment in childhood can sometimes grow up to inflict pain on others, but this can only ever explain their evil actions, not justify or condone them. We do not find out if anything has happened to Montressor that far back, only reading about some sort of imagined slight that happened more recently. However Montressor's characteristics come across as vindictive, analytical, narcissistic and methodical character traits. Also, he is obsessive as we see by his careful calculations about the interment at the end. Perhaps he has become over-sensitized to matters affronting his self-esteem, and that is why he responded in such an over the top way to a small insult.
Nothing can really justify out-and-out premeditated murder, and certainly this is true of Montressor's actions in Edgar Allan Poe's short story, "The Cask of Amontillado." We are never specifically told what act Fortunato has inflicted upon Montressor to cause such a horrific response. We can only assume that it was an insult of some sort, perhaps perpetrated at a much earlier time. Both Montressor and Fortunato are obviously of a higher and wealthier class than the average person, and family or personal honor seems to be at the heart of the matter. Certainly Montressor has planned his revenge well, and he succeeds with the perfect crime. But justification can never really be accorded him; his choice to take personal action instead of through lawful means makes him a murderer and a criminal--nothing more.
Montressor is very much like Fortunado in that he is arrogant. Fortunado is arrogant in that he succumbs to flattery about his knowledge of wines. Montressor's horrifying action stems from an insult to his reputation about being knowledgeable about wines that Fortunado made in a remark to others. Unfortunately for Fortunado, Montressoris a more diabolical person who can plot the demise of another and barely show any hesitation. He is educated in repartee as his constant gibes toward the unfortunate, drunken Fortunado. The plans are made for every possible event that might forestall his plan to do away with the man who insulted him. This egotistical, scheming murderer had no real justification for his actions. No one would ever know what happened to Fortunado except Montressor himself. It was not an act of self-defense or even an eye-for-an-eye revenge. It was arrogance of the utmost.