Violent video games can induce violence in other people, but that trigger would have to be there, first. I'm not gonna doubt, though, that video games CAN give people ideas. Also, people who think they are "training" by playing a game and then go out to imitate these actions are going to get a cruel realization...it's not the real thing.
Regardless, no one is making you buy these games. If you become enraged after playing a game, go get help. And there's no way that anyone under 18 can buy M rated games, just like anyone under 13 can't buy T rated games. It is never the game's fault, just ignorant parents and people who need help.