[Paleo Conservative]

He may have developed an obsession about it, and so become convinced that he did the murder. False confessors are often quite convincing - they have already convinced themeselves.

If he's a false confessor, how did he leave his DNA at the crime scene?

[Arec Barrwin]

To my knowledge, there is no evidence of a DNA match to date.

[BlackVeil]

From a recent news report:

U.S. and Thai officials did not directly answer a question at a news conference about whether there was DNA evidence connecting Karr to the crime.

The DNA evidence should settle the issue of his guilt, when it is available.