Why is everyone treating me this way? And why did I help?

I was in school when i hear a whole bunch of laughter. I go towards the laughter and i see these 3 guys picking on this small fat kid. They were playing keep away with his food and he tried to get it back but they threw ut in the trash. And then the kid got on his needs and started to cry hysterically. And then all the kids that were around started to laugh at him. When i saw this i don't know what happen to me but i found myself beating the Sh## out of the three bullies.Then when i came back from suspension everyone started to ask me why i beat them up saying it had nothing to do with me. Almost everyone in the school except a few close friends keep asking me this. All i want to know is why did i help? i don't even know the kid? and why do people see me as the bad guy?