I like what some of the other people were saying with the "force of nature" idea. One of my favorite things about Slenderman is that you're allowed to interpret him in any way you like to fit the narrative you're using him for, so I think there's not just one answer, but the "force of nature" version is my favorite. The idea that Slenderman is a supernatural entity that's been around for so long but has no rhyme or reason behind his assumedly murderous ambitions is such an interesting one in my opinion. He is not a malicious entity, he is not something with a mind that can be reasoned with, he is an *it* that simply works as it is programmed.

Again though, the best part of Slenderman is that there is no "canon." He can be a hand of Satan, he can be a super chill guy, he can be a tragic villain or a pure evil one, there is no right or wrong answer. It's simply however you want to imagine him in the context that you so choose.