Remembered another "game to blame" story. The washington sniper - there were news stories claiming that it might be a teenager, bent on Counter-Strike, taking the game to the masses. And would you believe it! It actually was a military trained marksman. What are the chances of that?!
I think the games can affect people who are already disturbed. I personally once tried to attract the cops in GTA vice city by shooting pedestrians from a multistorey car park. I found that my position gave me total invulnerability from the police and i was able to stay there shooting off limbs for a long time. And then i felt disgusted. For me hurting innocent people felt wrong even in the game. And generally i drove around pedestrians and avoided shooting anyone for no reason.
Another example was that in KOTOR i found it very difficult to make dark side choices, as even in the game i want to be good, help people and avoid unnecessary violence. I finished at the top of the light scale.
I think my upbringing is to blame for that. And in spite of playing a lot of violent games i still know whats good and whats evil.
As for Manhunt - I haven't played it, but read the reviews. They might've gone over the top there. But if it gets banned I am worried that every other game may have trouble getting through the censorship. Btw - iirc carmageddon was banned in brazil and in some countries people were replaced with green-blooded zombies.