Why would someone be obsessed with someone they hate?

I suppose it depends on what you mean by obsessed, what exactly they are obsessed with in terms of this person and how tehy handle it. Nothing is ever simple.

To feel an emotion that intense probably increases obsession chance same with love etc some just have obsessive personality

I call that love hate

Yes, it just shows how that "the opposite of love is indifference" is just an empty cliché

Idk some people are mad insane types