If it were deliberately done and didn't occur as a result of structural or equipment failure.
It looks to me that the Malaysian authorities really have known what happened since sometime on Tuesday and definitely by Wednesday, but it seemed so extreme that they didn't want to come out and say anything until they had agreement and consistency in agreement from other authorities. The "ping" satellite data confirmed their suppositions, because it confirms their radar track identifications.
There may have been a struggle in the cockpit which would account for the sudden altitude loss detected at one point. If someone realized what was being done, that person might have tried to seize control and bring the aircraft down to a low altitude per SOP so that the cabin air could repressurize from the outside. If so, it looks like that person lost his/her life in an attempt to save others.
If it had been an equipment or structural failure, the plane could not have been repressurized and was flying long enough at high enough altitudes to exhaust the cockpit air supplies. A knowledgeable person (probably the pilot) could have programmed the navigational sequence and left the piane flying ghost until it ditched when the fuel ran out, but the motive if the person were not guilty is hard to imagine, and it appears that someone with access to the cockpit deliberately started disengaging tracking systems in a planned sequence before the loss of contact.
This does not mean that pilot was guilty. A pilot would normally act to preserve the lives of the passengers, and even if the pilot did the course programming, the pilot may have been acting under duress. It's possible, for example, that the pilot may have programmed this course while telling the attacker(s) that it was a different course in an attempt to protect civilians in case he could not regain control of the plane. What's notable about the course trajectory is that there is very little time over land. They'd have been flying in the dark for hours, and by the time the attackers figured out what was up they might not have been able to recover, or even to figure out where they were.
Depressurizing the cabin wouldn't require much knowledge. Programming the course would.
I've written this highly speculative but possible scenario because I don't want to fuel slander of those who may have died heroes by pointing out the rather obvious - that the passengers must have been dead, or someone would have gotten a cell phone signal out over the ensuing hours, and that the initial altitude climb was probably an attempt to kill them rapidly.
We don't know who diverted the flight or why, but it cannot have been a simple "pilot suicide" scenario, because there was a clear attempt to buy time by evading detection and creating confusion and a pilot has no need for such elaborations if he wants to take a plane down. There probably was at least one attempt to wrest control back and save the passengers. If the pilot survived that attempt, I can easily imagine that he might switch his priorities to protecting potential civil targets by essentially putting the plane on a path to nowhere. In his grief and rage over the passengers' fate, he might not have cared much about his own life, and who can fairly argue that his priorities were wrong?
Or he may have programmed the course before then in an attempt to gain time, while telling the perps that he was programming a very different course. A western course flies into the night and maximizes flight time in the dark.
This is a very horrible incident, and care should be taken to avoid unfairly maligning the innocent. Eventually I do believe they will find the wreckage, but it may not tell them that much due to the hours of flight after the diversion.
= new reply since forum marked as read
