In "The Cask of Amontillado," Montresor is driven to revenge for an injury he never explains, nor does he provide any clues or innuendo.
Montresor does use Fortunato's inebriated condition to lure him through the tunnels. He also uses Fortunato's extensive knowledge of wines to win Fortunato's trust to accompany Montresor underground.
In the story, neither Fortunato's words nor behavior toward Montresor prove that Fortunato is worthy of hatred: there is nothing that I can find that would support Montresor's insistence that this man had "borne" him "a thousand injuries.
For example, when they meet, Fortunato is very friendly with Montresor, though the latter attributes this to Fortunato's drunkenness.
Fortunato can hardly believe that Montresor has a cask of this rare wine during the Carnival season, and he dismisses Luchesi's ability to discern the true quality of the amontillado. Still, neither of these things warrant Montresor's hatred.
Fortunato is easily duped into going with Montresor, but being foolish is not a reason to be hated. Fortunato does imply an insult (I think) when he alludes to being a freemason himself, while insisting that Montresor could not possibly be one as well. He sounds much the snob, but I still do not believe one would hate him for such a sentiment—and Montresor gives no indication that he is at all insulted.
I have always believed that Montresor is quite mad, and that the injuries he refers to are imagined. This would explain why there is nothing on Fortunato's part in the story to substantiate Montresor's bitter hatred toward him.
That's a tricky one. Personally, I think Montresor is clearly an "unreliable narrator" (suggestion: This is a common term used when discussing literature; Google it and get a good idea what it means).
Especially unreliable when it comes to Fortunato; it should be fairly clear that M.s thinking has something wrong with it. More precisely, I think he misinterprets what's happening in the world, and his own place in it.
Assuming M reports their conversation accurately, I don't see anything particularly hateful about F, (your own opinion may differ; read carefully and decide what you think) but clearly M does. Is he right or wrong? Does he have a disordered mind that interprets F as hateful, or is he right and merely giving an accurate report of the situation?
Not to give away any answers, as I said, I think M's mind is dangerously disordered: you could arguably say he is murderously insane. (These are very outdated terms; you could look up "paranoia", "schizophrenia" to get a sense of how M is thought of today).
The question now is, if M is essentially a crazy murderer, who likes to torture his victim, is he hateful? Or is he a victim of his own disorder, more properly to be kept away from harming people, but to be cared for rather than hated?
The reference I give is for Wikipedia, which is often suspect as a source of information. However, I think this article, and the others it refers to, are adequate.