I am of 2 minds. 1) Pilot suicide. Yet why would he fly it till it ran out of fuel instead of dumping it in the drink right away. 2) Rapid decompression putting the crew members to sleep. The Payne Steward affect if you will. They say the direction of the aircraft changed before the co-pilot said "All right, good night," If that is true it will remain a mystery for the ages. Ok 3) Close Encouters of the Thrid Kind. The plane will be found a few years from now in a desert in Mexico, fuel tanks full and all passengers with a sunburn on one half of their face.
Pilots should have had time to get their oxygen masks on and go into an emergency dive. If the plane did drop to 12000' they would have revived (assuming the Malaysian data is accurate). In addition it appears that the plane changed course to the south once they were over the Indian Ocean.
I was in the air in the midwest during the Steward incident, his plane never changed course and just kept flying until it ran out of fuel which is consistent with pilot incapacitation.