As much as it's all been said before, we're looking at this problem in entirely the wrong way.
What separates man from animals. More to the point, what separates man from predators?
Well, when you look at the average plains ape, we don't have a lot going for us.
Bipedal motion tends to be slower than quadrupedal and less stable.
As far as our senses go, we have piss poor night vision, no sense of smell to speak of and our hearing isn't that accurate either.
We have no fur or other hide to keep us warm, and no natural weapons.
We live in large groups and prefer the company of other our animals of our kind.
Do you know what this makes the average human? In the grand scale of things, it looks a lot like we're a prey animal.
The only thing a human has going for it, is the ability to reason and use tools. And we've sort of milked the last one dry haven't we?
So, our approach to Slendy has been as an "entity" of unspecified malevolence and provenance.
But we're doing it wrong. Because we're still thinking like apex predators when the position happens to be a rung higher on the ladder.
There's always a bigger fish.
Tall, dark and slender is an apex predator.
From what we can tell, it has the brute strength to rip apart a person limb from limb.
It has some of the most perfect camouflage we've ever seen.
It can drive a person stark raving mad.
It can move seemingly ignoring the normal laws of physics.
And it might just have a sense of humour.