In Which I Examine the Enigma Itself

Sorry about the little-more-of-Hamlet and less-than-Holden post last night - however, since I have no blog followers and this is more of a place to vent and write down my ideas, I won't bother removing it.

Now there is something that has been bothering me. For all you ARGers, all you internet junkies and horror fanatics, you'll know what I'm talking about. The Slender Man.

"We didn't want to go, we didn't want to kill them, but its persistent silence and outstretched arms horrified and comforted us at the same time..."

What is it that makes us all fear this being?

Slender Man first appeared on the Something Awful forums, for a "create a paranormal image" thread.
But what can turn a tall, eldrich horror from a kidnapper and an arsonist into something strange and unknown, something usually nonviolent but surrounded by AV distortion, operator symbols, and health issues as seen in alternate reality games like Marble Hornets and EverymanHYBRID? Very seldom is Slender Man seen as a violent being, with the exception of places like Just Another Fool where it's hinted that he has actually killed and kidnapped people.
Yet this abomination still strikes fear into all of our hearts. Sleepless nights are spent thinking of what he is, not that he'll come after you. A man burning down buildings, stealing your children, murdering you. That's scary. But what's wrong about some cosmic stalker? At his worst, he's creeping in your room like Entry 14 of MH or episode 6 of EmH. Maybe there's the occasional blog where he's portrayed as some malevolent evil, but those aren't the ones that gain fame. Infamy, maybe, but what people want is the mystery that he is.
And why do we fear him? We all know he doesn't exist. He's no serial killer or sex offender. He's not a great white shark, or a coyote, or a wildfire, or a flood. He isn't bankruptcy or failure or death of a loved one.
All us ARG-writers, we play along, knowing that what we write and read isn't real. But yet we're more shaken than any horror film could shake us, more disturbed than any dream could disturb us.
Why do we fear him? Because he is fear itself. We fear what we cannot see. What we do not know. Could he be an arsonist? A killer? Sure. But what if he was something worse?
What if the worst he could do wasn't drive you mad... what if he was nothing? Everything?

That's my question to the world. What is Slender Man?

I'm sure that if we knew the answer we'd all get a lot more sleep at night.