What if C and D talked to A about it instead of hurting him?

1) Person A Hits Innocent Person B

2) Person C Hits Guilty Person A for Hitting Innocent Person B

3) Person D Hits Guilty Person C for Hitting Guilty Person A

The way I see it, it's all about intention. C was sticking up for B by reprimanding A for doing something wrong to an INNOCENT person. D was reprimanding C for reprimanding a GUILTY person. Continuing Person D's logic would ensue a never-ending hit fest.

Using that scenario, I'd say C is in the right (although there were better alternatives) while D was not. This doesn't come down to science or facts but rather morals and values.

I would guess all of them are in the wrong. No harm had to be dealt by any of them, necessarily.