I strongly suspect there's a big correlation ≠ causation issue with "violent video games".
I'll wager that obsessively liking violent video games is a symptom of problems and not a cause. A normal person detaches from the violence somewhat, and just gets immersed in the goals and the challenge of the game play.
The troubled person gets obsessed with the killing, personalizes it. Revels in it. The game didn't make them that way. Although it may let them exercise thoughts or attitudes they already had.
And considering that the First Amendment issues with video game content are even more protected than the Second Amendment firearm ones, I don't think it's very useful avenue to pursue, even if I did think it was a "root cause" type of issue.
I'm with AJ on this one. Most non-violent people I know, while liking FPS games, don't revel in the violence part. I think the reveling in violent (games/movies/etc) is an EFFECT of being violent, in the same fashion kinky people like kinky pron, while kinky pron doesn't make non kinky people kinky.
Violent games/movies, etc are either seen as fiction by well adjusted folks, or fantasy as those less well adjusted.
Another side of it...I enjoy military thriller novels (red storm rising being my favorite, but include Clancy, coyle, brown, etc) but it isn't because I'm imagining myself as a character.
As for the mental part, calling this person autistic is sad...and a result of the whole spectrum argument advanced over the past few years. A truly autistic person would have never behaved this way, or likely even been capable of this behavior.
As someone who exhibits -some- classic high functioning autistic (aspy) behaviors, I am horrified to see this as a "reason" or excuse. Molon labe.