@cicerone imposter,
Until his shooting rampage, Lanza didn't display any anti-social or violent tendencies, at least none that people, outside his home, who had contact with him, were aware of.
But Val Killmore has a good point. The influence of violent video games, in terms of enhancing the appeal of violent fantasies, and stimulating such fantasies, particularly in one who is socially isolated, withdrawn, and otherwise relatively powerless in the world, and who might be playing them excessively, is certainly something that should be considered. The games may have given him an image of being powerful, and hyper-masculine, in a way that he could never achieve in daily life, and his mother then supplied him with the weapons that enabled him to live out that fantasy, and leave his mark, and that powerful image of himself, on the world. Particularly if he was very depressed, and possibly suicidal, over the limited sort of future he saw for himself, given his disabilities, that's a real possibility. This rampage could have been his swan song, his suicide, and the form it took could have been shaped by violent video games.
It's something that parents might want to think about if their children play these games--particularly if their children have significant problems with their social functioning, as this young man did.
The only main parenting mistake, that we know about, was not locking up those guns.
