I intentionally broke someone's heart and hate myself for it
Are you actually joking right now? What made you want to break their heart?
Because I'm selfish and just wants to hurt someone
It is sometimes necessary to break people's hearts to be able to move on. With your situation seems that you did it intentionally and just wanted to hurt them and you should avoid that, you should apologize the person and explain why you went this route with them. If you provide no explanation, the person is going to continue to be completely hurt and you will continue to feel bad.
Maybe I won't.
You shouldn't if you don't have to, because breaking someone's heart is something most people would agree should be avoided at all costs. If you do break their heart though, then apologize and move on. Let them get their closure and then find people who won't break their heart like you did.