The maneuvers, radio silence and timing of leaving Malaysian airspace all point to deliberate action. There are questions here like what happened to the cockpit crew.
As for the passengers, the most likely explanation I've seen is that the cabin was depressurized and hypoxia likely eliminated that problem.
Why? The most likely explanation is suicide-by-plane much like the Germanwings incident. It seems like this was even more premeditated such that a lot of effort was made to make sure the plane wasn't found, likely this was so the pilot's family would get a settlement as the pilot's actions couldn't be ruled a suicide without the black box.
Literally nothing else makes sense.
I don't know how long the black box can retain information, particularly given it's likely ~4km under the ocean and by now has been for years. It's a shame the extensive (and expensive) efforts to locate the wreck have all failed. The search area is remote and deep making search operations difficult and expensive.
This too seems to be by design.
It also seems quite likely that investigators having the pings from the plane to narrow down the search area was a flaw in that plan. The pilot was likely unaware of this possibility, which again is more evidence of deliberate action.
I do hope this can be settled one day.