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.
Slenderman is kind of interesting. The idea started in a thread on Something Awful, which is a comedy site (mostly juvenile humor) with a forum where a many users create interesting or funny Photoshop images. (I’m a longtime SA Forum guy myself)
Slenderman originated in a thread where users were making “paranormal” images. The Slenderman images were really creepy, and the forum users built up a backstory around the images. The stories included a lot of archetypes you see in myths and urban legends—children and families being stalked or abducted by supernatural entities, sinister “Men in Black” appearance, coverups by the government, people being driven crazy when they find out the truth about the creature, and so on.
A number of people have taken the idea of Slenderman and created their own drawings, photos, and videos (like Marble Hornets on Youtube). There’s even a Slendermen character in the game Minecraft.
It’s interesting how a story can take root and spread so easily. If you have access to the SA Forums, you can actually read how the character was created and developed as proof that Slenderman isn’t real. If you’re at all interested in myths or things like urban legends, Slenderman is actually an interesting example as to how stories like these arise.
Of course, there will always be crazy and disturbed people in the world. This girl, and apparently her family as well, are proof of that.