The contact made by the pilot of the other aircraft to MH370 wreaks of Hypoxia due to a cabin pressure incident - I'm actually stunned that this pilot didn't recognize this.
He's been asked to relay comms due to loss of communication with ATC, he establishes a line with them and instead of the appropriate response of "yeh all is good, we'll find another frequency, thanks for checkin' on us!" - he hears nonsensical mumbling and no satisfactory response or attempt to re-establish a better line. Their behavior was clearly reflective of functional incapacity due to advanced hypoxia (which would have been just minutes at the most before complete loss of consciousness occurred).
Who's to say that when this happened, one of the crew hasn't slumped forward into the control column, disconnecting the auto pilot due to the application of the required force and deflection coupled with the cabin pressure emergency - sending the aicraft into its final dive...