I've always had the feeling I'm a very bad person and it is terrifying me.
Why would you think you are a bad person?
I've acted very heartless and mean to others.
It's good that you understand that you need to make changes in your life. Recognizing that is the first step to making important changes that will help you to treat others better.
I like this side of you.
Like I said, acknowledging your faults is the first step to fixing yourself. If you find you're still struggling, I'd recommend seeking out professional help and talking to someone.