Hans Reiser could "plausibly deny" committing murder, except he had acquired reading material about how to commit a murder. That was enough to pressure him to enter a deal.
Not really. Reiser's problem was that, as a massive narcissist, he took the stand and went with the "you cannot convict me because even if I did it I am smarter than you" defence. He came across as an arrogant liar, so the jury convicted. We don't know what would have happened if he had the sense to STFU and let his attorney argue that all the evidence was circumstantial: the books weren't the only evidence the prosecution had.
He took a plea after conviction in exchange for showing where the body was buried.