According to Montresor, Fortunato is someone that deserves to die. Not really a lot of specific details are mentioned in the story, and it is a bit difficult to trust Montresor. I mean he lures a guy into his wine cellar to bury him alive and feels no remorse about it. That's a bit twisted.
There are some character traits that the reader can infer about Fortunato though. First, the guy has an addiction to wine. He's already quite drunk by the time Montresor approaches him, and Fortunato is hungry for more wine (especially if its a really good one as promised). Second, Fortunato is a trusting individual. If he wasn't he wouldn't have gone down into the cellar. Third, Fortunato is likely insensitive to the feelings of others.
THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.
I still think Montresor is crazy, but it is clear that Fortunato is a bit of an insensitive jerk. A thousand injuries he has done to Montresor. But then he finally insults Montresor, and that is the last straw. I'm telling you right now, I wouldn't be super nice to someone that has wronged me a thousand times. I would imagine that Montresor is the same way, but Fortunato is too insensitive to notice he is causing others pain, he's too insensitive to notice that Montresor is mad enough to kill him, and he's too insensitive to even pay a compliment to his host. He's just greedy for more wine.