I think it's never justified to smack someone.
But from my perspective of growing up (late teens) I admit I've found myself occasionally laying the smackdown. And felt the other people fully deserved it. It's kind of odd, since I truely feel physical agressiveness is never good for anything. But then again, especially immature people, simply don't listen to words. It feels like smacking them around is the only way to get them to pause and think about their actions.
Ofcourse, that's just wishful thinking, and smacking them around only makes them more angry and more willing to get the revenge.
But then, how do you get out of such a situation passively?
If you're cornered, the people won't listen, and they are prepared to smack you around, would you really let them? I wouldn't, I would smack right back.
It's easy to feel cornered when being a teen, much easier than it would be to corner me now. You got your reputation to consider. Don't want to be known as a coward in school, etc. So when someone challenged me in some way, I always accepted the challenge. Had I been the passive guy running away all the time, I'd probably be subject to psychological warfare. And it seemed that getting in an occasional fight, or at the least stand your ground, prevented that.
It's sad.
Anyways, no, never justified. But I can think of situations where the results of fighting are better than not fighting. >.>