The scenario that best fits the facts is a spontaneously initiated “drastic political protest” by the captain that went awry.
At long last, a plausible scenario of what happened to Flight 370 has emerged. By plausible I mean that the scenario fits all the known facts.
The key piece of evidence has finally been released by Malaysian authorities: Pilot Spoke to Air Controllers After Shutoff of Data System (NYT.com).
This proves that one of the pilots turned off the ACARS communications link and then reported to air traffic control (ATC) as if all was normal. Twelve minutes later, one of the pilots switched off the aircraft’s transponder, which transmits the aircraft’s altitude and location.
This sequence of events more or less proves that one of the pilots was in charge of the aircraft. Given the lack of evidence of duress, this sequence strongly suggests one of the pilots was executing a plan of his own rather than following orders of hijackers.
Given the strong political views of the captain and his mastery of the Boeing 777, all evidence points to the captain as the pilot who turned off the communication links and was in command of the aircraft thereafter.