MH370 hypothesis

1. Fire
fire accident or minor explosion in the cockpit, and render the plane to fly without ability to land or steer. The pilot decided to minimise the causualities of plane crash to populated ares, and set the plane to autopilot mode to direction where there is no man's land, just falling unconscious. Plane continues to fly due south to Indian Ocean, and deep dived after it run out of fuel.

2. Suicide
the plane was deliberately crashed by the pilot or co-pilot, similar to Lufthansa 9525

3 The plane was shot down

other theories on wikipedia