Justin Bieber because people are so envious of him.
anyone who has fans that devoted is the nemesis of everybody else.
Plus, many people just LIVE and LOVE to hate. They could really care less about the individual, but to fit in, they have to develop an interest in hating as well. It becomes fun at a point as well. Then when one becomes so famous, they become that much easier to target. If people REALLY hated Justin Bieber, his videos would not have over 1,000,000,000 views. It wouldn't even have 10,000. They love to add their little two-cents in to try to get favorability from all the other haters, to feel important. They are OBSESSED with half of the celebrities that they swear they hate.