I'm biased by empirical data - e.g. look at youtube comments (anonymous) vs. amazon reviews (real identity owned by amazon).

Human nature demonstrates that a fraction of people really like to put down / bully / see others suffering because they are personally unhappy (e.g. the shock talk shows). Anonymity allows bullying behavior with no consequence.

More simply, why do people spend time and energy online "trolling" others. Clearly a sizable portion of people like trolling others, though I don't personally see why.