Violent computer games are an easy and convenient scpaegoat for many of the problems in society, when the real blame lies with parents who don't teach their children right from wrong, schools being unwilling to discipline children (I'm not talking about bringing back the cane, just taking a firm line) for fear of reprisals, the prevailing gang culture, and a so-called "justice" system that hands out piffling penalties (if any) for wrongdoers.
Just like those lunatic parents in the US who seek to blame their kids' suicides on heavy rock bands supposedly putting satanic lyrics backwards on their records, the people seeking such bans need to look at the real reasons behind the problems, rather than settling for a nice easy target. As supernick so rightly says, children should not be playing these 18 certificate games anyway. If a parent allows a child to play such a game, it's hardly the game makers who are the irresponsible ones.