I wouldn't be a bit surprised if fps games were saving more lives than they take. They provide aggressive and irresponsible teenage boys with an outlet and catharsis while keeping them occupied for hours instead of out bored drinking and looking for trouble.

That said, there might be 5 or 6 of the most extreme sort of lunatics out there who could be effected, together with some other radical influences, to commit horrible acts.

People deemed "Mentally Ill" are, as a group, more peacable than the population in general. If you remove the small minority who are deemed MI AND are obviously violent toward other innocent people or obviously OBSESSED with gorey or mass mayhem imagery, the remaining MI population is FAR more non-violent than the pop as a whole. It would make FAR more sense to pull non-MI people off the streets at random and shoot them full of poison while imprisoned rather than bother the MI people who are not obviously obsessed with violent imagery or commiting violent acts against innocents.