tu don't really get to know him. tu don't know his life story like Katniss, tu don't know his personality, tu don't know much at all. That's why I don't like him. And maybe I would if i knew more, tu know?
As the others have said, I think he is mostly hated because he killed Rue. I understand that, but I personally think that he was just doing what they were all there to do. Only one person was supposed to make it out alive, so they were expected to kill eachother. Plus, I dont think they do this in the book, but in the movie they make it look like he was going for Katniss, and Rue just got caught in the crossfire.
Marvel is on of the careers meaning he`s in a group of killers and volonteered himself for the games. The carrers try to kill everyone thats why non of the tributes other than the other careers liked him.