I find it interesting that in that link I just posted, it's said that the biggest factor in this event, beyond even Slenderman, was the relationship the girls had with one another.
I truly think there is merit in looking at other cases where two young ones have teamed up to kill, in order to understand these kids better. What these professionals are saying is, there's a certain psychological 'chemistry' between some pairs of kids that provokes them to act out violent fantasies.
Oh -- just to add: they also use the word "isolation" a lot -- I can say this from experience, this doesn't *have to* mean the kids were left alone for long periods of time. I had a similar pair of teenagers spiralling into this sort of relationship in MY home. And I was *watching*. Thank goodness. I put an end to it, before it escalated. But my gut told me there was something not right about them "acting like twins" you know, that level of closeness. And I was right to do that.
I believe this can happen right under the noses of caring parents, because I've seen it happen in a far, far less harmful way.