He isn't. That's why he's not giving them a chance to retaliate or defend themselves.
Never. You are "guy who doesn't think Nazis deserve to die" every time you try and highroad someone in an ethical debate from now on.
The fact that there's twice as many as he needs means he ISN'T absolved though. If he kills the full 20k, then it's not just to revive his friends. It's because he's made the decision to utterly annihilate his attackers.