My current theory: at least two of the cops saw an opportunity to frame Steven Avery for Teresa Halbach's murder when one of them stumbled upon her vehicle. They planted evidence in her car and in Avery's house (at various points in the investigation), burned her corpse on his property and then moved it to his fire pit, then hid the car in a very easy to find location, and coerced a false confession from Brendan Avery. I'm not willing to go so far as so to say the police murdered Halbach themselves, though it seems pretty certain that it was in direct response to his lawsuit against the sheriff's office for 36 million.

I don't think Kratz (the prosecutor) is in on it, but all law enforcement personnel involved seem determined to make Avery responsible for the crime. It still blows my mind that Kratz was allowed to make a press conference detailing how Avery (supposedly) killed Halbach, based on the obviously false and coerced confession by Brendan, presented as fact. Everyone in the state was hearing this narrative for over a year before the trial; there's no way they could ever get an unbiased jury for the case.