So we have the following factors:
1) Transponder ceased working.
2) Aircraft turned.
3) Garbled radio message with another aircraft.
4) Possible fisherman sighting flying low.
5) Possible businessman sighting flying low.
6) USA P3 Orions searching Malacca Straight.
So something very traumatic was happening in the cockpit, but not enough to stop the plane flying at that time. Obviously at some stage it did stop but the potential search area is immense.
The traumatic cockpit even could be:
1) Cockpit fire or other major mechanical/electronic event.
2) Terrorism.
3) Crew suicide attempt.