To answer SIDA!'s question.
A. I think it's extremely unlikely that Avery or Dassey slit her throat in the trailer or shot her in the garage. Pretty close to zero percent chance.
B & C. I feel like I don't have nearly enough to go on to say whether Avery killed her some other way. My gut feeling from listening to him throughout the show is that he doesn't sound like a guilty man -- but my gut feelings are wrong very often, so I'd rather rely on physical evidence. Unfortunately, there are extreme problems with the main pieces of physical evidence (the car, the key, the bullet, the bones). There's all kinds of reasonable doubt, so as a juror I'd certainly vote 'not guilty,' but there are plenty of unexplained questions if he's actually innocent. Where did she go after she left his property? Who killed her? Why did Brenden act like he was hiding something even when he wasn't busy incriminating himself with obviously false confessions? Probably the two biggest factors that point in the "guilty" direction, for me, are (1) the judge pretty clearly thought he was guilty, and he heard all the evidence and presumably isn't as easy to fool as the jurors, and (2) Strang, at the end, seems pretty open to the possibility that Avery is factually guilty (even though he's adamant that he shouldn't have been found legally guilty). Suicide seems unlikely. I really doubt the cops killed her. (The only really bad police, it seemed, were Lenk and Colburn. But how would they have known that Halbach was going to Avery's property? I think it's very likely that they did their dirty work after her death, not before.) There are some other characters we don't know enough about to rule out -- her brother, Avery's brothers, her roommate, to name a few. But there's no good evidence against any of them. I think I'm stuck punting this question and going with 50% B and 50% C for now.