:: My guess- Homo who recently found out he has AIDS and can’t pass his upcoming flight physical ::
Possibly but why would he not simply rent a Piper and drive that into the ground? AIDS is, in the end, a personal “weapon” that does not require a multitude of lives lost to make a statement.
My money is still on the possibility that the other crew member in the cockpit was found to be “involved” with the other-member’s significant other (wife, concubine, boytoy, etc. - and I’m gonna stick with the heterosexual angle for now).
That would explain locking the crew-member out.
Or his advances were turned down by pilot