Just an act of random pilotage?
I suppose it happens.
I believe such events of known random pilotage suicide have happened at least 3 times in history with moslem virgin seeking martys at the controls. There is historical precedent. Theory cannot be discounted, and should be at the top of the list—certainly a highly likely scenario.
What about insurance fraud? I wonder if the pilot or copilot had a large life insurance policy. I mean, it seems it was a deliberate attempt to evade radar, and the plane is ditched in a very remote part of the ocean. If not for the engine pings (which almost nobody including the pilot knew about), it seems that the chances of finding the wreckage, or any information would be impossible other than small parts.
I don’t know if it would be possible to jump out of a 777 plane at low level/speed with a parachute. Could he arranged to have a boat pick him up out there?