This started off as a fascinating insight in to a horrific crime and it's young instigators but it is far too insular in its focus.
By the 60 minute mark I was satisfied that the assailants motivations had been biasely justified, the whole "slenderman" and mythology in general had been thoroughly covered and I was craving for the story to expand in to the physiology of the young impressionable mind, into psychopathy (a blatantly obvious trait of at least one of the girls) and for the focus to switch to the victim and her family (in a respectful way, considering they didn't want to be involved), their complete ommission from the documentary plays out as an indifference to their horrific experience instead.
I was also surprised with the multiple mentions of "believers" needing a group to belong to and the brainwashing involved in that dynamic, that the documentary makers weren't brave enough to bring up religion in that discussion (especially as a world leading expert on the subject Richard Dawkins was on a Skype call at one point)... But no they only used santa clause and the tooth fairy as examples.