Well, there is some truth to the idea...
Think about it, when you first watched someone's head explode in a movie or something, it repulsed you. (At least I hope it did), but after seeing it over and over, you don't mind it anymore. For someone with preexisting mental instability, watching video game people getting killed over and over just might desensitize you to killing, add mental instability into the mix and the lines get blurred. "Wouldn't that be fun to do in real life"...