There are screwed up people everywhere in all walks of life and they go off the rails from time to time. Per capita of world population, they are in the minority, but they make the headlines.
If one of said screwed up people happens to like RPGs or Heavy Metal or Acid Punk or decides to pattern the killing spree on a movie that seems to reflect how (s)he personally feels (and felt long before the movie came along) or something else that "safely" puts the person into some minority pigeonhole and makes him "one of them" so far as society is concerned, then the movie, the music, the RPG gets blamed and people say "it made him/her do it".
They conveniently ignore all the hundreds, thousands, millions of others who listened to the same music or played the RPG or watched the movie as they are still a minority and can be safely marginalised for the "greater good" of making the rest of society feel superior.
So long as Joe and Jane Average can say "oh, that's one of them, it could never possibly happen to us", society as a whole is happy - they can act out their personal prejudices and bigotry, safe in the knowledge that they are "better" than someone.
And if you can cash in on that and turn a quick buck in the process or grab your 15 minutes of fame as the moral crusader who "knew all along that this stuff was dangerous", then all to the better, eh.
Ever notice that if it's a supermarket clerk who has led an otherwise dull and humdrum life that goes of the rails, people never turn around and blame working in a supermarket for his behaviour...
Sadly, it's impossible to beat because there are never going to be headlines/10-o'clock-news stories along the lines of "And again today, 10,000 RPGers had a quiet time and didn't commit suicide or slaughter anyone, that makes this the 10th outbreak of exemplary behaviour in the last month."
Frankly, we're not newsworthy enough for our natures to be widely known.