Any one remember Helios Airways Flight 522?
Or Payne Stewart's ill fated flight?
In both cases cabin pressure was lost slowly, most would just fall asleep, those still awake would do odd things, not make good decisions.
I.e. "all right, good night", changing course for no good reason, shut off tracking stystem.
On the Helios flight, alerts(lights and bells) went off but flight crew was already effected so thought it was false alarms and turned them off.
Pilots last contact with ground was because of an alarm, the ground engineer asked him directly "Can you confirm that the pressurization panel is set to AUTO?" , pilot disregarded the question and replied "Where are my equipment cooling circuit breakers?", this was the last communication with the flight.
Had he checked the pressurization panel, it was on manual, and if he set it to AUTO, pressure would have been restored.
In both cases the aircraft flew until out of fuel and then crashed.
My theory is this(the above), or a similar type incident, is what happened to flight 370.
Sudden depressurization is not good but is better than slow, with sudden you still have your wits about you and can react with good judgement.
On Commercial aircraft at least one pilot is suppose to be wearing an oxygen mask when above 12,000ft, BUT, there is also a statement that the oxygen mask must be easy to put on and near by, not worn, so........for sudden depressurization not slow.
According to facts I have seen.
Flight took off at 12:30am local
Did the odd turn and "all right, good night" about an hour into the flight, so definitely above 20,000ft for enough time to start falling asleep and have poor judgement, like shutting off tracking equipment, maybe because alarms were going off, person in cockpit just shut everything off so he could sleep.
The airplane sent off a ping to a GSO(geosynchronous orbit) satellite every hour, last ping at 8am, airplane had 7 to 8 hours of fuel on board, so that fits with running out of fuel in the following hour before next ping was due at about 9am