Here are some of my wilder theories:
1. A mass suicide. Everyone onboard actually got together to commit suicide.
2. Someone(s) was onboard who belonged to some government agency (those two men with fake passports?), got in some kind of trouble, and the plane may have actually been found but is covered up by authorities for whatever reason.
3. Some kind of human trafficking ring that tried to smuggle all those people.
Now here are a couple serious theories:
1. I actually like the accidental one the HotD posted before, involving some kind of electrical fire or whatever and the pilot turning back.
2. However, I do believe there was some kind of malicious intent by one or more people onboard. It then involves the phenomena of the "graveyard spiral." A lot of airplane disappearances involve the graveyard spiral. Someone not trained (or with very little training and just nerves) tried to get a hold of the plane after the hijackers were apprehended. Maybe the pilots were killed or injured during the hijacking. Regardless, someone who didn't know how to fly the plane tried to fly the plane. Graveyard spirals are known to make one go so far off the map because you think you're going in one direction but you're actually spiraling in a wide circle down to oblivion. This could be due to many conditions. Weather (Although it's been said it was clear), sensory illusion, panic, etc.

Since it was much bigger than a biplane, it could have spiraled, straightened out, spiraled again, etc, ect. Going wildly off the map and crashing at a very high speed into oblivion.