[A pair of 12 year-old girls stabbed a friend nearly to death purportedly in order to please or placate Slenderman. Other posters were describing the girls as "monsters." ]

The demonic "Slender man" appearing in the girl's dream was the monster - and yes, that experience really can and does happen to people. The children are spiritually and mentally victimized, and you, like a "genius" label THEM as monsters? They're human children. Do you even know what influences played into those evil and horror things they got caught up in?
Didn't think so.
There's no "excuse" for what was done - it was a heinous and especially vile evil thing that they did - but at least know yourself first. Because if there is one thing that I have seen time and time again, this sort of accusation which denies the humanity of human beings comes out from or into a particular living manifestation:
1) Either the evil they accuse others of being is what they are guilty of themselves
2) or still are partaking of (unchecked)
3) or will be otherwise led to partake of by that same REAL monster.
A soul that judges another (beyond the action to the PERSON themselves) in such a manner will have much to suffer themselves for the sake of remedial reparation (and no small building of the virtue of humility necessary for salvation) if they are not repentant early on. Innocents have as much to suffer because of the increasing ignorance of the Truth of Christ and God in the increasingly atheistic "humane" (merely neo-Paganized, really)