bootfa:
Maybe just maybe
He had an argument with the F/O that caused the F/O to
get so angry that he locked him out of the cockpit, when
he went out to relieve himself.
Then, maybe just maybe, the Captain got so angry
about being locked out that he went berserk.
I believe the NTSB got control of the DCVR, which is probably a two hour recording, at least.