They should be properly controlled. That's what the rating system is for. Personally I would never want my 12-year-old cousing playing an M game, but if you let your kids play it, its not that hard to go with them and buy it. The rating system is SUPPOSED to only prevent kids from buying too-mature games - it doesn't stop the parents from buying it for them. And if that's what the parents want to do, so be it.
No, it is the moral decision of the person buying the game. Like... fireworks are dangerous but they are enjoyed by a lot of people therefore it's the person's choice whether they put their safety at risk or avoid them completely. Games and other entertainment perhaps should be treated in the same manner :-)