who does Buck hate and why?

Buck's hatred for Spitz begins when Curly is killed by the Husky and Spitz laughs.

"On Buck's first day on the Dyea beach he sees that he has been flung from civilization into confusion and chaos. He must always be working and alert among the "savages" who know only the law of "club and fang." Attack is a constant threat. The good-natured Curly is killed simply for trying to make friendly advances towards a husky dog only half her size. The dog takes her down with efficient, quick technique. Once it is certain that Curly will not rise again, the rest of the dogs in the camp attack and kill her. Spitz, the lead dog of Buck's team, laughs. Buck hates him bitterly. He has learned that fair play does not exist, and he resolves that he will never go down."

curly is killed by buck?

No, Curly is killed by a husky and Buck hates Spitz because Spitz laughs when Curlee is killed.