We have no evidence that Montresor has sufficient justification for the level of his intense hatred of Fortunato. First, Montresor is not specific about what Fortunato has done to earn his loathing. All he states, in the first sentence of the story, is:
The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge.
We get a clue in the story that Fortunato thinks himself superior to Montresor. Fortunato is a mason—he belongs to the Society of Freemasons—and is very surprised when Montresor says he is a mason too. Fortunato responds:
You? Impossible! A mason?
This could be interpreted as a snobbish response, showing that Fortunato doesn't believe Montresor is worthy of being part of the same organization he belongs to. (Montresor, in fact, is not a Freemason. He is making a pun on the word mason: he means he will soon be a wallbuilder, walling in Fortunato.)
We can see why Montresor might be irritated at a person who takes such a superior attitude to him and acts as if he is the better person. However, nothing Montresor tells us or that Fortunato does can lead us to conclude Montresor is justified in cruelly walling him into a cold, damp catacomb to slowly starve to death. It is hard to imagine what Fortunato could possibly have done to warrant such a response. This leads us to conclude that Montresor is mentally unbalanced rather than that Fortunato is deserving of his fate.