My wife and I binged this over Christmas. Have you guys read the post-Serial interviews with Jay? The purported rationale behind Jay's constantly shifting story is that he was holding back in order to not implicate himself or (more importantly) other people, but as he got more confident that he wouldn't be punished, his story became more truthful. It sounds weird, but that actually makes sense to me. The serial killer arc is interesting, but ultimately vacuous because if that were the case, it makes no sense why Jay would've framed Adnan when he (Jay) wasn't actually involved at all, why and how he would've made up such elaborate details, and how he would've known about stuff like where Hae's car was. When you boil it down, it has to be either Adnan or Jay, and while the podcast definitely biases the listener toward suspecting Jay, I think neither solution really makes more sense than the other.

Maybe they both did it?