Also like two weeks ago I noted hypoxia as the probable cause, which ChrisV and others shot down, for some good reasons. I don't know if I could put a percentage on what I thought the chances of inadvertent hypoxia versus planned suicide are. But for all the "you can't examine the state of mind of a guy trying to commit suicide" memes here, I noted that the exact same is true for a cabin that is slowly becoming depressurized. The pilots will start to act completely irrationally as their brain receives less oxygen.
When
these guys crashed, the NTSB blamed fatigue for the pilots: 1. not checking their instruments and 2. not alerting ground control that they were losing control during landing. The effects of hypoxia would be much worse than fatigue.
Brains without oxygen are fickle. They won't operate rationally either. Accidental hypoxia due to some kind of non-obvious depressurization would explain why one pilot seemingly didn't put up a fight while the other intentionally depressurized the cabin (assuming that's what happened). Or, it doesn't explain it so much as it could potentially rule out the pilot intentionally depressurizing or crashing the plane.