The mystery on this flight is astounding. No matter what theory you have, each theory cannot make sense which is why its so baffling. Here are the problems with each theory:
1) Straight forward pilot error or plane malfunction. If that is the case, then why not radio a distress or mayday message? Why did they change course after turning off the transponder?
2) Explosion on board. If that is the case, why did the plan keep flying for a while after the pilot turned off the transponder? The US military satellites also would have been able to pick up any mid air explosion.
3) Pilot suicide. If so, why fly for several hours, why not just dive straight into the ocean - why no suicide note or message? Also, there doesn't seem to be any indication that the pilots were depressed or had any terrorist connections or malice in general.
4) Hijacking - by someone else on the plane. If so, why no terrorist organizations have claimed it was their work? If it was for ransom, why no ransom demands?
I have heard other theories as well, but again, none of them would make sense based on the information provided.
My only strong suspicion is that the public is not being presented with all the facts or possibly some of the facts presented are not accurate because not one scenario seems to make any logical sense.