I really don't understand how anyone can blame this horrendous crime on anyone except the clearly psychotic and disturbed preteen girls.
Slenderman is old news (he has his own video game, for f***'s sake) and this is just evidence that there are far too many unsupervised kids being exposed to content that they cannot handle or easily distinguish as fiction and not reality.
A large population of the gaming community was around when Super Mario Bros., Doom, Wolfenstien, Silent Hill, Resident Evil, Mortal Kombat, and Custard's Revenge was a thing and all that it has done to most of them is to develop a strong admiration to an entertainment medium that was not widely regarded as viable for a majority of its existence.
People need to seriously grow a pair of whatever makes them take charge of serious situations like this and either institutionalize mentally unstable kids like these two and never allow them back into society again or, to avoid any risk, lobotomize them and avoid all the danger. I know it's extreme, but seriously, we have to make a stand that crazy is dangerous and if they are not kept in check, more innocent people are going to pay the price for their mental instabilities.