two wrongs don't make a right.
I actually feel for the bigger kid. I was bullied a fair bit in high school & I understand what it's like to be fed up, to have one too much and just snap. It was probably a case of the straw that broke the camels back.
Like I said, two wrongs don't make a right. My above comment is no way saying that what he did was Okay. Due to the context though it's understandable.
I'm disappointed that this forum would be a venue to promote such rubbish