I feel the hate is for fans and not celeb alone.
Fans talk a certain way and hence others get angry.
Sometimes what is written is just in retaliation to fans.
Also when it comes to celebs- especially from films or sports, people have tendency to compare. They will bash everyone else. Or somehow belittle them.
I have heard fans of MGR and Sivaji Ganesan used to have violent clashes and even killed each other.
In some cases behaviour or crime committed might also make people hate a celeb.
In real life nobody bothers much. Online people talk too much especially as there is no moderation and no ramifications for what one types.