I remember watching the documentaries about him in the 90's. As I understand it, there was scant technical evidence against him but he produced some very detailed and convincing confessions of his murders. One which he retold in an interview was him sneaking up on a camping German couple sleeping in a tent and stabbing them both to death while they were still trapped inside. He also told stories about his step father taking him on "hunting trips" in the woods in which he forced Quick to perform oral sex on him. Those stories were also found out to be lies but were also incredibly believable.
As I remember it, these stories were mostly due to the "psychologists" basically cheering him on, same with the media. This is an example of our abhorrent mental health system as much as our legal system.
Didn't they also manage to pull in some medium somehow?