"Smoke filled the cabin and overwhelmed and distracted the passengers and cabin crew ... or the cockpit door was locked and/or the cockpit was filled with smoke, so no one could enter the cockpit to try to figure out where the plane was, how the pilots were, or how the plane might be successfully landed. (This would be a complicated task, even if one knew the pilots were unconscious and had access to the cockpit, especially if most of the plane's electrical systems were switched off or damaged).
With no one awake to instruct the autopilot to land, the plane kept flying on its last programmed course ... right over Pulau Langkawi and out over the Indian Ocean. The engine-update system kept "pinging" the satellite. Eventually, six or seven hours after the incident, the plane ran out of fuel and crashed.
This theory fits the facts. It makes sense. It explains the manual course change as well as the "pings" that a satellite kept hearing from the plane. It requires no fantastically brilliant pre-planning or execution or motives."