First off, both the man and the kid were in the wrong here.
The kid shouldn't have been trash-talking, and even though he's 13, he still had the logic not to do something like that. But does that really mean he deserved to be assaulted or worse? And the man completely lost his temper, even with a history of mental problems. You don't invade someones house and strangle them over a video game.
Also, why all the hate for the kids mother? Did anyone consider that maybe she was in another room, and came in when the man started strangling her kid? Therefore not knowing what the kid was saying?
And the "that kid shouldn't have been playing CoD anyway, its rated M" thing? It's not illegal to play an 18+ game if your under that age, its a recommendation. So the mother obviously thought he was mature enough to play such a game.