Only David would quote an informant as reliable.
This should be against the law.
It doesn’t even sound the way Stephen would speak. His vocabulary is very limited.
Its reliable because it matches the physical evidence and contains the same things Brendan told the police. For example, that this was his first sexual experience.
Ask yourself this - What other feasible way is there for Joseph Evans to obtain this information while in Jail other than from Steven Avery?
Do you really think he got hold of Brendans interview videos and Steven Averys court documents and carefully put all this together? Why bother he has nothing to gain from doing this. Avery talked about his crime and this man is trying to warn the public about the TV show.
Anyway. There is no question to Avery's guilt and the proof is all there for you to see. If you dont want to see it then that is down to you.
There are many small clues in his behaviour also.
Here he is asked when he last used his burn pit. He stays silent for 30 seconds and cant bring himself to admit he had a bonfire after Teresa vanished. He then lies saying he last used it before Teresa arrived.
https://streamable.com/zm6q0In the early days of the missing persons investigation when people were still hoping to find Teresa alive and BEFORE any crime was reported he was already claiming the police were framing him. Framing him for what? He knows Teresa is dead because he killed her. He knows police will uncover evidence against him and he is already using this conspiracy allegation because he knows he has been caught.
https://streamable.com/wh9ph