For some reason people assume all creepypastas live in a Mansion all happy singing koom ba ya. Well, for starters, where the heck did a Mansion come from?! All jokes aside, though, anyone reading this theory, it's just a theory. So, DON'T. FREAK. OUT. I call this the Puppet theory.
Here is a theory, what if all the creepypastas ranging from Sally to Masky and Hoodie are....dead? Slenderman (just assuming it's correct) can get into people's heads. Slenderman is usually near children or young adults (sometimes). What if he lures his victims or fellow creepypastas ranging from 1-30 years old to use them? Meaning, what if he lures them to drain their energy. Or controls them to do his dirty work. Think about proxies, they are shells of their former self. If Slenderman didn't have proxies we would see sightings of him everywhere, causing panic possibly. It's too risky for him. People would hunt him down trying to put him down. So, he uses proxies to do a few jobs for him than drains their energy, killing them. Similar to what ghosts do to children. You see, some ghosts drain energy from kids so they can communicate with the living. But in Slenderman's case the energy could be useful to him so he can teleport around and such.
That's were Slender Sickness comes in, Slenderman wouldn't just take in a lot of energy that fast. So, he abducts kids (because they have more energy than adults) and keeps them alive, at least until he gets hungry. That's why his victims are never seen again....they are dead. (Or in some other place where we would never find the.) And those lucky enough to not be taken by him get Slender Sickness, weakening them so they give in. (Or ya know, he doesn't want people figuring out his little game so he weakens them as a warning.) Slenderman is the puppetmaster, his victims and proxies are puppets.
Do you believe this is possible? It's a bit of a long stretch though, I must admit.
'Till next time-L.V