Here's my take on this:
It's human nature to want to explore humanity's dark side. Long before there were video games, there have been books, plays, operas, music, etc. about things that are "bad". It's the reason everyone cheers for Richard III in Shakespeare's play of the same name. It's the reason American Psycho got turned into a movie. This whole ESRB thing is about as productive as banning books was.
If anything, I'd rather have people actualizing their desire to experience the "dark side" by playing games. At the end of the day, at least games never hurt anyone. True, gaming seems to be a popular scapegoat these days, but the fact is, games don't cause violence anymore than Marilyn Manson caused Columbine (something else the pundits tried to allege not too long ago). It's very easy to observe A and B and say that A caused B, but it's also a serious logical fallacy.
At the same time, I don't think every game is appropriate for everyone. Specifically, if you've got kids that are really into guns and violence and think the answer to all their problems is to shoot the bad guys, then they shouldn't be playing games that reinforce that belief. If a parent hasn't properly taught a child that real violence hurts real people and has real consequences, then that child shouldn't be playing games that glorify violence. Parental involvement, and not a bunch of letters on game cases, is what keeps kids on the right path.