None of that explains why a plane with all aboard dead would do a 180 degree turn in flight.
Hypoxia causes increased breathing rate, headache, lightheadedness, dizziness, tingling or warm sensations, sweating, poor coordination, impaired judgment, tunnel vision, and euphoria, depending on the person.
Short of evidence to the contrary, and awaiting confirmation, it would appear that the pilot was (along with the rest of the folks on the plane) already suffering from hypoxia before they arrived, and managed to turn the plane toward 'home' and set the autopilot.
The plane crashed as it ran out of fuel.
Impaired judgement and confusion could account for the reversal of heading. If that is the case, had they descended and landed at their original destination, chances are they would have made it.
This is just speculation on my part.
I will wait for the results of investigations.