“She found herself nevertheless a little wary of this strange little man who managed somehow to combine ages-old wisdom with an almost childlike curiosity.”—Conundrum by Steve Lyons (via heartoftardis)
Slenderman is actually The Doctor, when something went wrong with his regeneration. Physically, this resulted in his faceless, strange appearance. However, it also effected him mentally.
The Doctor is an explorer and observer, and as Slenderman he continues to do this. However, he is unable to remember the TARDIS, and so simply wanders the world and simply watches. He watches people, he watches things. If he feels threatened, he will act on animal instinct and attack.
However, the disappearing children (and sometimes people) are not often his victims. As we know, The Doctor likes travelling with an entourage, and it isn’t often that he is without companions. This want resides in Slenderman, although in a very primitive form, and results in him “collecting” people, often with their consent - sometimes without. He has never directly caused these people any harm, although many have died by now for various reasons.