Whether you think children should be allowed to play violent games or not, does this justify outright banning of all those games and their production?
We don't ban alcohol or cigarettes even though they are also illegal for under age people. This is quite an overreaction.
Sure kids can get 18+ rated games just as they can get alcohol, cigarettes and porn. That's where they parents come in, not the state.
EDIT: I agree that killing someone is never ok, it can be justified(self defense etc.), but that doesn't make it ok.