Why does Montresor hate Fortunato?
I do not think that Montresor hates Fortunato. I have never gotten the feeling that Montresor feels much of anything. I believe that Montresor is or is very close to being a full blown psychopath.
Psychopaths, on the other hand, are unable to form emotional attachments or feel real empathy with others, although they often have disarming or even charming personalities. Psychopaths are very manipulative and can easily gain people’s trust. . . When committing crimes, psychopaths carefully plan out every detail in advance and often have contingency plans in place. Unlike their sociopathic counterparts, psychopathic criminals are cool, calm, and meticulous. Their crimes, whether violent or non-violent, will be highly organized and generally offer few clues for authorities to pursue.
The above description from Psychology Today describes Montresor to near perfection. He's cold, calculating, and emotionally void while luring Fortunato to his death. He also feels zero remorse after the deed has been done.
Poe and/or Montresor never gives an exact reason for why Montresor wants to kill Fortunato. The closest the reader gets to a reason is stated in the first sentence.
THE thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge.
Montresor is likely using hyperbole to describe Fortunato. It's not likely that Fortunato hurt Montresor 1000 times. Despite that though, Montresor doesn't have a problem with it. But when Fortunato insulted Montresor one time, that was enough to push him to seek revenge. The insult doesn't seem to have sparked anger or hate. The insult seems to be the final wrong that deserves to be punished . . . by death.