Time for me to play devil's advocate here and work with the remaining areas of doubt.
1. A person with a mental-health issue may also have any sort of physical ailment, even a young one, which is totally unrelated to the mental-health issue. Narcolepsy, for example, or uncontrolled hypoglycemia.
2. Could it be possible that SOP, for at least the captain and/or copilot on this flight, was to always select "LOCK" when one pilot left the cockpit? If so, then obviously the pilot still in the cockpit would have to remain conscious. It would seem dangerous for this to be SOP. Do any pilots do this? Anyone here?
3. Robin said something to the effect that Lubitz started acting strange partway through the flight - delayed answers - disconnected.
4. Is it possible that whatever this physical ailment may have been, or medicines prescribed for it, resulted in some sort of confusion/psychosis/loss-of-consciousness, possibly drug-induced, which had been coming on since the beginning of the flight, that led him to think that somehow he was alone in the cockpit, and had to dial in an emergency descent? More specifically, could it be that it was normal for them to LOCK the door, but that Lubitz was quickly sliding into some sort of altered mental state, and then passed out, unable to respond to the captain, with the door still in LOCK mode? We have not heard how long the captain was out of the cockpit before attempting reentry. Maybe the FO was about to pass out, had been feeling increasingly bad but was trying to hold it together in front of the captain, and then when the captain left he lost it, went into confusion, saw no other pilot beside him, and set an emergency descent, confused and not thinking clearly about the terrain ahead thinking he was trying to save the plane, and then passed out? (Remember that the descent started over water.) Pilots in hypoxia have done some very strange things. But so could a pilot about to lose consciousness from a medical reason, maybe even from some medicine he took. Sometimes a new medicine can have a drastic effect on one patient, where it wouldn't badly affect another. What are the chances that the doctor who wrote the "do not work" note, also prescribed something, possibly something new?
Yes, I know that's all low percentage. But, for the sake of fairness, I wanted to work with the last bit of daylight between murder, and something very unfortunate happening to the FO, which led to a terrible accident. I am uncomfortable calling this murder or suicide just yet. We still have an unknown, in my opinion. This is where video footage of the cockpit, or running biometrics on the pilots, just as they do with astronauts, is critically missing. It may become key to determine what medicines Lubitz was on, and whether the doctor prescribed him something new.
[Edited 2015-03-27 12:30:16]