Occam's Razor suggests you are correct. Hijacker takes control, turns off transponder, reduces altitude to avoid radar, deviates flight plan direction, fight to regain control ensues (however many minutes later) and plane is driven into land or water along the deviated course. Conjecture, yes, but the most likely explanation is a hijacking followed by a crash.
I agree that’s what happened. There is a witness on an oil rig. I am finding it harder and harder to believe that the Malaysians did not have contact with the hijackers.
“fight to regain control ensues”
That could be it. If passengers knew there was a take over, they had hours to figure out a plan to respond. Like Todd Beamer.