I think we can pretty much rule out this being an act of organized terrorism or a hijacking. There would have been a claim of responsibility if it were a terrorist attack, and there would have been demands and a landing if it were a hijacking. So, we've got a few possibilities:
1.) A crazy person (not part of a terrorist organization) blew up the plane mid-flight, perhaps over-doing it a bit by bringing enough explosives to take out an aircraft carrier.
2.) One of the pilots just put the stick down for whatever reason (depression, religion, schizophrenia, hallucinogens).
3.) Pilot disorientation leading to a crash. Perhaps a loss of instruments in the middle of the night over the ocean.
4.) Sudden breakup due to some sort of mechanical or structural issue.
5.) Some sort of slow, unnoticeable fatal issue, such as the gradual depressurization scenario described earlier.
6.) Some country's Air Force shot the plane down without realizing it was a passenger jet, and is a bit embarrassed.
The biggest problem we have now though is all of the conflicting location possibilities. At this point, we're all reasonably confident that this plane is somewhere in a triangle between Madagascar, the Eastern Himilayas, and Fiji.