> Why is punishment administered by the state acceptable but punishment
> administered by an individual, not?
For one thing, that law is fairly representative of a civil society; kind of like stop signs. Sure, if you come across one at 3am and no one is around, you might be tempted to blow it off. But if everyone did that, we'd live in chaos.
It's not so much the punishment that I think the State ought to have a monopoly on, it's the decision between guilty and not guilty. This setup is pretty much what we've all agreed to; yes, it is broken in some significant ways, and yes sometimes it's particularly broken for something that happens to you and yours.
But without it, we're sunk. So yes: I'm very much against your friend's ex-husband beating the crap out of this guy. Sorry.
/jordan