I hate his character, seriously..but as for why it's kinda hard to explain.
Spoiler for spoilers:
First of all, I hate the way he acts like HE IS NOT EVERYONES boss and goes on about that friendship crap when in fact he is just psychologically controlling them. (vampires)
Second, he said himself that he has an evil character...but no, you are not evil you just have major issues.
Third, I hate the way he uses Zero.
Fourth, for forcing Yuuki to be his "lover" in return for her past.
Fifth, he is sneaky schemer.
Sixth, he bitch slaps Aidou a lot
Ah anyway, he is just a very unlikable character.