In the case of Eric Harris and Dylan Klebold, Doom was to blame for their violent and destructive tendencies. After the Columbine shootings rocked the popular consciousness, everyone was looking for answers and Doom was a favourite of both of the shooters. Unlike most of the cases, Doom played a significant role in the lives of both Eric and Dylan.
Eric Harris created several custom fan-levels for the game as well as a few for Quake as well. His most infamous creation, "U.A.C. Labs" is still in distribution including violent, juvenile messages hidden within the text files from Harris himself. Maybe the NRA's got a leg to stand on with this one.