@Maximus is right. The documentary is one sided to present a story about how the evidence is tainted. There is some meat on that bone for sure but Avery's guilt/innocence is not the focus of the story.
I have talked to 2 people, 1 guy from Green Bay and another from a surrounding community to Manitowoc. Both were emphatic that he did it and he's a bad dude. They said we saw about 60% of the evidence and another 40% that was pretty damning wasn't even addressed. The documentary also didn't offer up another plausible scenario of how she died or who else could have done it.
The thing I found most disturbing was the kid's confession. That was sad to watch. He didn't know what he was doing, couldn't comprehend the gravity of the situation and just wanted to please these guys who were out for blood.
One thing I didn't understand was how the prosecutor argued successfully in a court of law 2 different versions of the same crime. For Avery he argued that she was killed in the garage by Avery alone while for Dassey he argued she was shackled to Avery's bed in his trailer and had her throat cut there.