After watching the series, reading a great deal about the case, and thinking about it...
I'm unsure about Steven Avery. I really have no idea. I want to feel that he's innocent, and I certainly think that whatever actually happened, that the police doctored the evidence to make sure he didn't beat the charges. That being said, this is a man who was already clearly victimized by the system once, and it's easy to see how it could have occurred again.
I am 100% positive that Brendan Dassey did not have the mental capacity to have committed the crimes for which he has been convicted. It's unconscionable that the prosecution against Dassey went forward at all, given the glaring issues with his confession, the unbelievable actions of his original attorney, and his capacity to understand the situation he was facing. They basically made one case against Steven Avery, and then turned around and made a completely different case against Brendan Dassey--regardless of the glaring factual issues at hand. The story Dassey was systematically coerced into presenting had no basis in any physical evidence found in the Avery trailer. The taped conversations with his family once he was in prison showed he had no grasp of what was happening to him. (The fact that he continually thought that telling the police what they wanted to hear would allow him to go home, and heck, the whole thing about asking his mom to tape Wrestlemania for him, shows that he was completely clueless as to what was happening.) The testimony did not match. The evidence that was presented didn't add up, and his defense team did little to combat it. And the fact that the prosecution had already gone public with explicit, purported details of the case that were, in fact, complete fabrications didn't do him any favors. He didn't have a chance.
In the end, if there's anything we can take from both this case and the case of Adnan Syed (of the NPR podcast Serial), it's that all of us should thank God every night that we're not facing prosecution in the American legal system--especially those of us who may be at an economic, social, or mental disadvantage. The pressure for prosecutors to procure convictions will always win over any reasonable defense an innocent and disadvantaged person may present. And that's scary.