I believe that violence has some utility value within this age group, primarily it teaches a basic lesson.......getting hit hurts, and hitting hurts other people. You will be upset after you hit someone. Life isn't a video game; striking flesh has real and painful results.
That being said, in today's atmosphere a child who hits someone can get into a lot of trouble.......and carrying this behavior into situations as an adult can ruin your life forever.
I teach my kid(s) never to hit anyone for any reason. In a self defensive situation, I presume that a survival instinct will supersede my policy and my kid will protect herself. That may seem odd, I know.....but I can't trust my kid to determine when it is OK to use violence so I depend on our natural programming to aid the decision.
If this kid were mine, I would punish him; even if he was justified. I would not want him to think it is OK to punch your way out of a situation.