HOLY SHIT THIS FUCKING SHOW.
Alright, so I was wondering why Marty was lying for Rust about how all that went down and now we know. Obviously Reggie Ledoux was not the serial killer, but he was awful. It's understandable that Rust and Marty would think they'd really captured the killer after finding those two kids, but the camera spent a long time making sure we knew Reggie didn't have the spiral tattoo on his back.
I do not think Rust is the killer, and I don't think Marty is either. I think Rust is still, after all these years, working the case. They haven't shown us why Rust left the police force yet, but I'm betting he has been obsessed with the case ever since that guy's confession and apparent suicide. IMO Marty's daughter is going to be the key to solving the case. They've shown us several times that something happened to her when she was a little kid. First with the dolls, then with the pictures she drew, and now she's captain of the varsity slut team.
I bet the serial killer is that sleazy preacher, or he is involved somehow.