Psychology
A man whose wife has just announced that their marriage is over, that she is leaving and taking his 3 beloved children with her. This man, a normally decent individual, is boiling with rage, frustration, humiliation and loss. His life as he sees it is over - pointless. His anger immense.
The next day, his first alone, he is on the flight. People are behaving normally. It enrages him more. He is not thinking rationally, the rage is all-consuming. He decides.
His plan is simple enough. At the appropriate point, go invisible, fly back over Malaysia and then take the plane to 45,000 feet. He doesn't want to die by smashing the plane into the water. Lack of oxygen is an easier death.
And he doesn't want it to look like suicide. He programs a flight path into the autopilot, and hopes that the plane will return from 45,000ft and follow that path until it flames out in the middle of the ocean, never to be found.
He will be missed - oh yes - his family will cry for him. They never should have left.