Paranoia

That kid: the one whose girlfriend seemed to have become a Slender Man victim? He decided to attack some cops.

Apparently, he was screaming about how they were puppets of "him" and that they'd infiltrated every level of society and how he was going to eradicate them all. It seems that he'd cracked and was convinced everyone but him was a proxy.

Anyway, the cops fought back. He's now in the hospital.