I think Thompson severely overstates the effect of video games relative to other influences, but insofar as they're part of a culture that associates violence with joy, yeah, they probably have some effect. People are produced by a culture as much as they produce it. Where Thompson becomes an insufferable jerk is that he doesn't consider any other influences. He takes a cop killer and reduces their lives to one factor: they played GTA. It's ridiculous.
I do think it's weird, though, that people who get the most out of video games, people who will talk glowingly of the immersive qualities of video games, who will spend hours playing them, who develop fond memories of events that occured inside the game, nonetheless reject the notion that those experiences could also affect them in a negative way. I think video games have tremendous positive potential to help us reconceptualize who we are and frankly, our place in the world. But, I think we should recognize that there's a negative potential as well.