Small childen should not be exposed to anything graphic: Richard Ramirez was a kid, not even a teenager when he used to hang out with an uncle who would tell him stories about killing people in vietnam, and shot his girlfriend in the face right in front of him. Fucked him up but good.
Now, exposure to some of these things at an early age can lead to an unhealthy obsession with the subject.
However, do i think letting a 14 year old play doom and watch hellraiser will inspire him to shoot up his classroom? Not unless the little fucker wa seriously unhinged to begin with.
On topic, a bit:
a little article I wrote for PLanetHalflife during my tenure as a staff writer there:
http://planethalflife.gamespy.com/Vi...s.Detail&id=22