I know that the theory that the pilot (i.e. the Captain) did it is the most plausible, but I have so many questions regarding not just WHY but HOW.
The pilot suicide cases I know of are:
The Germanwings co-pilot Andreas Lubitz on flight 9525. He waited until the Captain went to the toilet and then locked the cockpit and plunged the plane into a mountain range shortly after. The Captain was trying to break down the door to the cockpit to stop him, but because Andreas Lubitz crashed the plane quickly, the Captain didn't have time.
The co-pilot Gameel El-Batouti on EgyptAir flight 990 (although I know there is still slight dispute over this). He demanded to replace the original co-pilot who was meant to fly and then he also waited for the Captain to go to the bathroom. As soon as the Captain left, he switched off the autopilot and electronic controls and plunged the plane into the sea. The Captain tried to take back control of the plane, but again because the decent was so quick, the Captain couldn't stop it.
So in both cases the planes were crashes very quickly preventing the second pilot or crew from being able to do anything.
In the case of MH370, the plane flew for another 8 hours off course. How is it possible that the Captain could just calmly fly the plane for a whole 8 hours in the opposite direction without the co-pilot or crew being able to do anything? It's a very risky plan.
Also, in both of my above examples people were not surprised that those pilots did this and whereas here those who have worked with the Captain in Malaysian Airlines were all adamant he didn't do it. Even the stewardess in the Netflix documentary who lost her husband in MH370. Why would he suddenly out of the blue plan such an elaborate heinous act after an exemplary career and seemingly no personal problems?
I'm clutching at straws, but is there any possibility at all that there could have been some mechanical issue of the plane and the pilot was trying to return the flight back to the original airport but couldn't?
It's also an insane coincidence that a very safe large airline lost two passenger planes within the space of 4 months! It's almost like someone was trying to bring down the airline.