As I’ve read here, I agree with the theory that the plane was hijacked, probably by the pilot, and probably with the intent to fly back to Malaysia and crash into something governmental.
I strongly suspect, in the process of turning around and hiding his course, the Chinese shot the plane down because it flew over something sensitive.
The Chinese freaked about shooting down an airliner and, now that the situation has gone on so long, will never say a word.
So far as we know, the plane never came within 500 miles of China.