They believed this story, which is idiotic in the first place. So they went and stabbed another human being to "please" him. Wow.
Not only did they believe the story, but they didn't even believe the correct story.
Here's Slenderman in full:
He's tall, well dressed, slender, and has no face.
Don't look at him, bad things happen, like indescribable death.
Can't run.
???
Nothing else. No 'cult leadership'. No 'romance'. No 'kill people to please him'. He is in no way within the same universe as any other creepypasta character. There is no 'Slendergal'.
I didn't even find Slenderman interesting in the first place. But all these 12 year olds drove the entire thing straight through the Earth. It's irredeemable if you've seen some stuff. Avoid DeviantArt at all costs.