I don't know if Shawn is responsible but I can't categorically dismiss his horror fascinations as having nothing to do with this case.
It's hard not to think of Mackenzie Cowell and Chris Wilson, the guy in jail charged with murdering Mackenzie. Wilson was fascinated with gory stuff too and the account (told by someone who knows him) of Wilson mutilating bodies at the funeral home where he worked reminds me of Billie's brother saying Shawn cut up deer with a chainsaw. If those stories are true, they are real-world manifestations of the horror obsession. And in Wilson's case, he was sadly willing to take his obsession to the next level.
Another thing about Shawn that really disturbed me is his obvious insincerity and smugness in the aftermath of Hailey's disappearance. I posted at the time that he reminded me of Eddie Haskell. For folks who don't know, Eddie Haskell was teenage a character on the TV show Leave It To Beaver who was really full of himself. He would put on an overly polite act when talking to adults while acting like a creep when he was hanging around with other kids. Just seems like there's a real disconnect with Shawn's personas. Seems like this also manifested itself with him raving about one of his horror masks on the Michael Myers forum a day after he had asked for those forums members' prayers for the missing Hailey (which was a little weird in itself, since he wasn't really what I would consider a regular poster on that forum). He had also posted on that forum in the past that he broke a girl's jaw who broke his heart.
I'm not that far above 50% on Shawn's responsibility (I'm not that confident in the police work in this case either) but I do think there's some troubling stuff surrounding him (and Billie). If he is responsible, I think he's probably too cautious to hurt anybody again anytime soon but I would be scared for people around him as time goes on and the heat really dies down.