I don't think anyone's arguing that the game's going to make you insane.
Psychopaths aren't insane. They just lack empathy. Not all psychopaths turn out as serial killers or child molesters. Some become lawyers and CEOs - trades were a lack of empathy is often considered a boon. Just that it "might" give an insane person the wrong ideas.
But Psychopaths already have the wrong ideas. It not like you can cure a brain damaged individual (or make it worse by playing video games). If they snap, all you can do is get the hell out of their way until the police put them down. but in most "civilized" countries criminals are NOT executed by someone with a large garden shears maniacally cutting at throats, stabbing, slicing, etc.
No. Instead we keep them locked up in small cages for 9 years (on average) after which we kill them in a way that is not even suitable for the euthanasia of pets. What difference does it make to the person getting killed whether he was in excruciating pain because he's having his head severed with garden shears, or being paralyzed with Pancuronium bromide after which a dose of Potassium chloride induces cardiac arrest. Both methods are equally traumatic (to the person being murdered) and ultimately fatal. That someone would advocate some method of murder they find more aesthetically pleasing is the kind of thing that would keep you awake at night. I guess it comes down to personal taste; for some this sort of "pretend" violence is considered extremely distasteful.
For some people the idea of abortion is considered extremely distasteful, yet killing doctors is just fine and dandy. These are the kinds of people who pose a threat to society (or if you don't care about society, then you personally), the people who find ways to justify real violence, not those who enjoy pretending. Similarly it wouldn't cause any real agony if you were to watch (and enjoy) a video of a kitten being brutally tortured, for example, yet this is still considered extremely distasteful.
I thought the Bonzai Kitten hoax was hilarious. It was even more hilarious reading the hatemail from all the tools who thought it was real (never mind possible).