Religious texts, advertisements, and tall tales have been "egging people on" to do things they otherwise would not for thousands of years. Judas Priest and Internet message boards are not new in this respect.

There is something unique, I think, about these message boards, though. I think most people would agree, the overwhelming majority of humans are reasonably decent—they're not going to murder others, are not overtly racist, etc. And throughout human history, these truly 1-in-a-million horrible people have been isolated, unable to find fellow horribles to befriend and egg each other on. The internet has changed that. No matter how unusual or awful your interests may be, it's trivial to find another group of people who share in these thoughts and interests. I don't know what the solution is, or if there even is one, but this does feel like something truly new.