You know, when a 25-26 year old male becomes increasingly rigid rather than more nuanced in his beliefs, is increasingly belligerent towards "others", is suddenly nomadic and self-isolating when he used to be social and outgoing, is privy to "hidden" information and/or knowledge that no one else seems to "get", and he shows increasingly paranoid behaviors with apparent grandiose delusions, I typically suspect he's had a sudden break from reality. His reported highly anxious demeanor, his likely peeing his pants, and his erratic and irrational behaviors following the crime (i.e. holding onto all of the evidence), is also atypical. It makes me suspect he could have recent onset schizophrenia.
Not trying to look for something to excuse what he did - schizophrenia isn't a disqualifier from being prosecuted for murder as far as I know - but man he sure went downhill fast.
JMO.