I beat up the school bully so why do I feel bad

This guy who is 16 is a bully he has bullied me in the past but I never cared I am and it was the lunch period I was walking in the hallway and saw him hurting these 2 people and I see he is bullying my girlfriend and best friend he was holding both of them by the throat and slammed them against the lockers and they each started crying I rush up to him I pull them away from him and shove him he walks up to me and says"What you think you bad huh do something then do somethin"he punches me in the chest then in the face my body moved on its own and hit him in the chest then he punched my face and I started blacking out and thought to myself "Really I am gonna get knocked out this fast"I black out then the next thing I know I am being pulled away from he by 7 people and still (keep in mind I am not that strong)I see him leaning up against the lockers blood coming from his nose and mouth his faced bruised up his hand gripping his stomach crying I turn to see my girlfriend and best friend pulling my arm devastated and horrified I get RPC for a week and when I come back I found out he stopped going to the school people saying how cool I am but when I try to talk to my GF and bestfriend they both look at me like they don't even know me or who I am any more its now summer and I can barely say a sentence to them and visa verse and why do I feel bad for taking down the school bully I stopped him from hurting other people so why should I care what I did to him and why should I feel bad and how can I approach my girlfriend and best friend and have a real conversation with them