I'm not going to post any spoilers, but they believe he wasn't the nicest of characters.... He did some pretty awful things. A major rub against him is also that he is another demon, and some fans think he is basically "competition" for Sebastian. The hate, in all seriousness, is basically the manifestation of immaturity and intense feelings of Kuroshitsuji fans. The hate shouldn't bother you, because as long as you like Claude's character, that's all that matters. Your opinion is yours, and no one else's feelings should change it.