Today a new detail - the last communication was saying goodbye to Malaysian ATC, but it never said hello to Vietnamese ATC. Furthermore the last ATC Radar contact was at that handover area. After that nothing, except a possible Malay Military radar contact.
So at that handover point the plane stopped all radio comms and stopped all transponder signals.
We don't know about ACARS aircraft maintenance signals because cryptically they are not telling us, but if they did have ACARS data they would know the plane's later movements, and it is clear that they do not. Therefore we can assume the ACARS went offline at the same time.
So it could have been an accident - e.g. cockpit fire - or a deliberate switch off. Were it to have been deliberate then switching off at the handover point would have been the perfect place, because in the confusion of handover no one was suspecting anything for an hour or so.