“He was a quiet man.”
Exactly.
Warning signs of mental illness:
Recent social withdrawal and loss of interest in others.
An unusual drop in functioning, especially at school or work, such as quitting sports, failing in school, or difficulty performing familiar tasks.
Problems with concentration, memory, or logical thought and speech that are hard to explain.
Heightened sensitivity to sights, sounds, smells or touch; avoidance of over-stimulating situations.
Loss of initiative or desire to participate in any activity; apathy.
A vague feeling of being disconnected from oneself or one’s surroundings; a sense of unreality.
Unusual or exaggerated beliefs about personal powers to understand meanings or influence events; illogical or “magical” thinking typical of childhood in an adult.
Fear or suspiciousness of others or a strong nervous feeling.
Uncharacteristic, peculiar behavior.
Dramatic sleep and appetite changes or deterioration in personal hygiene.
Rapid or dramatic shifts in feelings or “mood swings.”
Source: Psychiatry.org
Beware of a silent dog and still water.
Though I thought of one thing this morning. What if he was hypnotized and coded? It is known that hypnosis cannot be used to force people to kill themselves, but it is possible to force them to do something that they think is harmless. For example: "Once the trigger event happens, you lock the door and set the autopilot on descent. Do not touch controls further. Close your eyes and relax."
This theory may have value because even a suicidal person is probably not too happy to set the course for his destruction and then wait for ten minutes. It wouldn't be pleasant. He could simply turn the engines off, for example, or to set the airplane to dive; probably it would have fallen apart from excessive airspeed even before hitting the ground - and the destruction would be just as quick.
This theory of hypnosis may also do something with the lost Indonesian airliner. Behavior of the pilot(s) there is equally strange. Were these incidents a trial run?