Which one is the right? "Why are you hating me" or "Why are you hating on me"?
Hate on is, in my experience, very much slang. It has the meaning to bestow hatred upon [someone/something], generally through insult or critique. If you're looking to use the idiom in this way, then the correct sentence would be:
Why are you hating on me?
If you are trying to ask why someone dislikes you and want to use standard English, however, then you probably want to ask:
Why do you hate me?
"Hating me" and "Hating on me" have two different meanings.
"Hating me" simply relates to how a person feels about you - generally reflecting an intense dislike but not necessarily consciously acting on that feeling.
"Hating on me" refers to their actions in relation to their feelings about you. For example, if they are acting on their hatred for you by purposefully making hateful or damaging remarks about you or acting in a way that is intended to cause reputational or physical damage to you.