Teresa Halbach uncovered a drug ring
In this scenario, Halbach wasn't just a photographer but a plucky reporter who stumbled onto a huge drug expose. DennaAbusesKvothe theorizes that she might've had photos of a local "marijuana field" under the protection of crooked cops, who may have started leaving her threatening voicemails. But when she didn't take the hint, they took her out -- and they also trashed her camera, in order to preserve that murder-inducing field o' weed.
Satanists did it
Why Satanists? Why not! Those guys love causing trouble around Halloween. "Something satanism-related," burningsodium writes. "And the guilty parties are either members of law enforcement, or else have connections to it. Really the least-complicated answer. Just consider the date of the murder."
Actually, a Satanic sex cult did it
This theory posits that members of a sex club with possible Satanic or Masonic roots killed Halbach in a ritual slaying. And because some of them are also cops, they framed Steven Avery as a bonus. It's all derived from a three-part YouTube confessional from a man who's had a lot of alleged experience with the group. As zoso471 posits, "Sacrificing Theresa [sic] AND getting to incriminate Steve Avery is a pretty attractive deal for those cops a part of this club."