Why do I behave like this? (Kudos if you read the whole thing)?

It's a possibility that you're insecure or paranoid, since you mentioned that you feel like you have to hurt people first before they hurt you. And you mentioned that people used to walk all over you, so that could also contribute to the whole insecure/paranoid thing. It could also just be that you have anger management problems and low tolerance for things. It could be a disorder/psychological problem too. You can talk to professionals to get this sorted out. I can't tell ya much. But talking to a psychologist or therapist is a lot better than Y!A. Lol. But I must say that I agree with you. Humans are selfish creatures in general.