I'm a prime example for why Thompson-ism is wrong.
I'm 20. I love splatstick (i.e. humouristic splatter movies) and played my first violent video game (Doom) when I went to elementary school (I nicked it off a warez CD of my uncle).
However in real life I'm a pacifist. I haven't started a fight in my entire life and never got beaten up or injured anyone (physically, at least -- animals included).
I also tend to have a high feeling of social responsibility on the personal level -- I'm the kind of person that helps old women across the street if they ask him.
Wonder why?
Well, I was raised in the spirit of pacifism and that violence is never an option and so on and so forth. That didn't harm me, but I don't think that's why I tend to be such a nice guy.
I would rather say it's because I've been raised to be responsible and consider the consequences of my actions.
I can't just start a fight and beat someone up at random because not only is that a crime, but the consequences of that very act would be almost entirely negative.
If I went to jail, that would have a negative effect on my future options (career, etc).
If I got the reputation of being a choleric brute, that would harm my social life.
If I killed someone (human or animal), they as an individual would cease to exist -- which may be a nice thought until you consider that their ceasing of existence can only mean further trouble down the road (most blatantly: someone who liked them could swear revenge).
And that's only the RATIONAL reasons not to do bad things.
If you add the ethics on which the concept of responsibility is based to that mix, you got a flat-out instant pacifist.
Now, why do I like splatter movies then? Simple: I like violence.
Why do I like violent video games? Simple: I can enjoy the violence without having to deal with real-life consequences.
Now, in most good RPGs everything has a consequence (given that you don't hack or cheat) -- which is why I most oftenly follow the path of the lone heroe and only start getting violent if I get frustrated and the game itself does not provide any further potential for satisfaction.
I wouldn't go so far as to say that video games can't affect your child. You have, however, to teach your child to deal with video games responsibly -- video games are not REAL.
The tiny bit you must not forget to mention when doing that, however, is that in multiplayer games, or on the internet, or in the news there ARE real people involved.
Other players in multiplayer games are real people (given that they aren't bots) and those people who are reported dead in the news are real people too.
That's a difficult concept to grasp, because it seemingly contradicts the simpler idea that TV = fake.
You need to make sure that by teaching the difference between reality and fiction you don't destroy your child's empathy.
It's a hard job and you can fail on so many levels, even by doing the right things, but that's the job you chose by becoming a parent.
Sidenote: Be prepared or you might find yourself speechless when trying to explain that kind of concepts. It's not quite as easy as it seems when you have to explain it in full depth to someone totally new to the whole idea.