From Business Insider - I cannot vouch for the publication or the theory but it's interesting:
Shortly after takeoff, as Malaysia 370 was flying out over the ocean, just after the co-pilot gave his final "Good night" sign-off to Malaysia air traffic control, smoke began filling the cockpit, perhaps from a tire on the front landing gear that had ignited on takeoff.
The captain immediately did exactly what he had been trained to do: turn the plane toward the closest airport so he could land.
The closest appropriate airport was called Pulau Langkawi. It had a massive 13,000-foot runway. The captain programmed the destination into the flight computer. The autopilot turned the plane west and put it on a course right for the runway (the same heading the plane turned to).
The captain and co-pilot tried to find the source of the smoke and fire. They switched off electrical "busses" to try to isolate it, in the process turning off systems like the transponder and ACARs automated update system (but not, presumably, the autopilot, which was flying the plane). They did not issue a distress call, because in a midair emergency your priorities are "aviate, navigate, communicate" — in that order. But smoke soon filled the cockpit and overwhelmed them (a tire fire could do this). The pilots passed out or died.
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.
Read more:
Malaysia Plane Fire - Business Insider