This basically.
They already have psych problems in the first place. I find the idea that Halo per say pushed him to going on a killing spree incredulous. Rather his parents denying him something he wanted coupled with him knowing where I firearm was.
A previous employer of mine shared the idea that video games are evil and ruin little kids. I've always been around them and my little brothers play them tons. It really isn't a big deal unless they are getting in the way of things that the person needs to be doing. (i.e. studying, working, helping their family, etc.)
Violence in video games takes a back sit for detriment in comparison to how addictive a game is. MMORPGs would be pretty harmless if they could be played in moderation. Speaking from experience here. The problem is they can't be. They're too addictive and they end up taking over real life responsibilities.