Proof that Slenderman is an excuse, not a motive
The girls said they were sacrificing Peyton and becoming proxies to save their families from Slenderman, right? Well, they didn't succeed in sacrificing Peyton or becoming proxies and yet were not concerned at all about whether their families are now at the mercy of Slenderman. I mean, if you honestly thought your family was in danger and the only way to save them was by murder, wouldn't you be freaking out after you found out you failed? They KNEW there was no danger and that's why there is no concern for their family. It's all an act. I don't care what they are diagnosed with, the fact that they planned a murder, AND a defense, proves that they were fully aware of what they were doing. I think the only factor any diagnosis plays in this is in giving them the delusion that they could pull it off.
Unless Peyton can testify that they knew Slenderman wasn't real, I'd assume the state will cut a deal with Anissa to testify to this against Morgan. We'll see.