One of the early reports, since quashed by the US media and downplayed by a few other overseas outlets is that the pilot, the one with the simulator in his bedroom, was a political activist in the Malay opposition and very active in his local mosque and had recently had some domestic problems with his wife. These issues were Never discounted by the Malaysian authorities.
My theory is that he compromised the on board systems, possibly disabling the co-pilot in the bargain, and took the plane off route in a way so tracking was limited, which he would know, maybe with the intent of making a political and / or religious statement, but then, later, something didn't go as he had planned, perhaps including the co-pilot regaining his senses and possibly attempting to retake control of the aircraft, and the thing went nose first into the sea at speed.
The resulting debris field would be a LOT of very small pieces, which by now may have dispersed to be almost unrecognizable.
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I saw this after I wrote that, and now, I'm not going to change any of it.
21 March 2014
Revealed: The transcript of the final 54 minutes of communication from the flight deck aboard missing Malaysia Airlines plane MH370
Communication from take off to the point of disappearance is revealed
The transcript shows the sharp westwards turn came as the cockpit handed over from Malaysian air traffic controllers to the Vietnamese
Expert said that would be the moment he would choose to steal a plane
Former pilot Stephen Buzdygan said: 'It was the only time during the flight they would maybe not have been able to be seen from the ground'
http://www.dailymail.co.uk/news/article-2586330/Revealed-The-transcript-final-54-minutes-communication-flight-deck-aboard-missing-Malaysia-Airlines-plane-MH370.html