nitpicker330 F14 highly unlikely the FO or Captain couldn't get back in. Without going into details you can gain access UNLESS the person inside doesn't want you to.
The people that need to know the code, know the code.
Exactly so. Too many suspicious pieces of evidence add upto hijack but more probably pilot suicide here -
(1) No checking in with Vietnamese after clearing Malaysian airspace
(2) Transponder and ACARS turned off at this point
(3) Turning off track
(4) No Mayday call
(5) Possible height excursion
Whether one of the pilots has fire-axed the other or locked him out and depressurised the aircraft remains to be seen. I would be astonished if this is not a criminal act.