First, the guy's 17. Give him a break, the game isn't even that violent. The online games are filled with 10 year olds for christ sake. Their crappy parenting probably caused him to be mentally incapable of being denied anything, therefore causing him to be a rebel and buy the game anyways.
Second, what a fucking idiot. It's a fucking game, and he goes and shoots his mom over it? I hate the way he did it too. Forcing his parents to close their eyes, doing it execution style, and telling his sister not to come in. This kid is 17, and that's close enough to 18 for me. Give him the god damn death sentence, or at least life. No less.
This incident was the result of a bad mix of too much parenting pressure, and lack of any maturity in the guy at all. It really looks like everyone was at fault.