If he was taking the whole plane down to make the ex suffer for breaking it off with him— he’d have made it clear to her that’s why he did it. There would have been a note or something.
It doesn’t seem to fit these facts.
You are using “western” logic. Remember, the captain was Malay, and “loss of face” is devastating. All of the machinations of the company and government have been dictated by this philosophy. Does Air Malaysia keep a handgun in the cabin like some U.S. carriers do now? Pilot 86s the co-pilot, heads up to fl460 which takes care of the passengers and rest of crew, sets the auto-pilot for the Roaring 40s, takes off his mask and goes to sleep. Hard to prove suicide at those depths.