I would think video games where you shoot up and kill people by the 100s for hours and days at a time. I wish they would show on the news the results of shootings and such, not pretty but when it is cartoon and sanitized its not real.
Movies. The basketball diaries movie started this crap. leo dicrapio walks into a school and shoots his classmates.
The two a holes that did columbine videoed themselves saying they hope a movie is made and speilberg directs it.