Yeah, it's a true mystery. The recovered wreckage seems to suggest a remote Indian Ocean crash site and I don't think there were any other plausible explanations for why the plane diverted so drastically from it's intended course outside of human intervention.
Malfunctions haven't produced this type of diversion before or since.
Multiple dispositions possibly point to what is essentially unknowable.
Why did the plane deviate from its course with no alarms? With multiple crew in the cabin, who among them would have missed the plane's course adjustment? Why did they miss the adjustment? If they didn't fail to notice it, did they cooperate, knowingly? If not, why were alternate communications not used? Were the people responsible incapacitated? If they were incapacitated, how did that go unnoticed in general, and cause further alarm, and alternate communications? Could there have been a threat of violence?
Basically, what I'm driving at is that one might expect at least one stray cell phone call in distress while over land, even if abrupt and short, with no discernable information conveyed.
Taking into account an air tight disappearance, it doesn't reflect the type of lone wolf depressive suicide like the European incident that crashed into the mountain. Other options include: botched hijacking that failed to ransom hostages; or sledgehammer as fly swatter, with possibly an entire plane destroyed to target an individual of interest, signs pointing to state actors.
1. The pilot waited until they we're alone in the cockpit then disabled all communications and locked the door. Possibly with a fake story over the intercom that kept people unsuspicious until away from land (diverting around some bad weather, etc.)
2. The pilot killed the co-pilot and then diverted the plane, same as above.
With a hijacking/ransom, the perps want to communicate, so I think that's out.
State actors is the only other one that makes sense to me, but it's still a murder-suicide by human hand.
Malfunctions haven't produced this type of diversion before or since.