What should I do? I feel awful about beating someone up badly.
Could you not control yourself from beating this guy up?
The stupid guy was harassing me from school that I had no choice but to beat him up.
You are nice to feel that it was wrong. Beating someone up is not right. They could have serious injuries, then you will be put in jail.
I'm just sick of the troublesome people in school
It seems that you were provoked at school and felt the need to stand up for yourself. While protecting yourself is okay, it's not okay to go overboard and badly injur someone. You should use this as a lesson to control your anger and try being assertive and not aggressive.