A bunch of people playing with madness.
And then somehow they are surprised by the outcome when mayhem ensues.
Slenderman is nothing more than a internet version of a scary old timey campfire story.
No doubt the mentally ill of the past have done bad things inthe name of the boogieman and the wickerman whatever from campfire sotries of the past.
The mentally ill gravitate to such stories of lore and will in some cases act them out with disasterous results.
Nothing new really just the medium has changed.