Yeah it's not, this is someone whose body and probably brain was eating him alive and he made it worse by self medicating with drugs and alcohol. I sort of believe his story, i know it sounds ridiculous but his health in the leadup and that he apparently didn't drink or take drugs that often beforehand is a pretty compelling reason to believe that his brain was melting. I think that's why they offered him a plea deal to ensure he got life, a good defence lawyer could've made a compelling case that he should've got a lesser sentence.
It's sounds like death would have been the sentence maybe quicker without prison. If cancer was in his brain or his body was failing him and even if he was insane technically he'd need medical and supervision. Prisons sometimes have decent healthcare. He was in a federal prison?