Yeah, I fully believe that the police involved in the blatant shenanigans do truly believe that Avery is the killer, and that by making sure he went away that they were doing the right thing by breaking rules to make sure it happened.
They're not evil or trying consciously to put an innocent man away or let a real killer remain free... They're just none too bright, and got sucked into a ridiculous situation because someone else (the killer) got the ball rolling by planting some evidence on an easy target.
Its definitely how I would try to get away with murder
. Find a creepy guy with a questionable past, someone that people will WANT to believe is guilty, and make sure the spotlight falls on him.
Not that I do much killing... .... ...