As I have stated before in my posts, I have been involved in lots of forensic work in my career. The basic principles of inquiry are:
1. Observe behavior that differs from the norm; and
2. Find out who benefits.
When Karr was arrested by the Thai police for suspicion of usinig child porn, I asked the same 2 questions about his confession and I concluded that both Karr and the Boulder DA would benefit from the confession. Karr benefited because he got the extradition from the Thai authorities that he wanted. Thai justice is not traditional U.S. justice. I am sure he knew that he could have faced something like a Turkish prison for his crime in Thailand. The Boulder DA would benefit because of obvious reasons.
As for "behavior that differs from the norm", Karr's confession and demeanor were an immediate tell. The Boulder DA not getting a DNA sample of Karr before the publication of his confession was abnormal prosecutorial procedure.
Karr is definintely smarter than the average bear. He got released by the Thai authorities and at the same time exposed the incompetence of our law enforcement system. It is a shame that the guy in Midnight Express did not confess to the Lindberg kidnapping. He might have avoided all that torture by the Turks.