Part of the mystery is due to the radical change in the flight path. It was supposed to fly North from Kuala Lumpur to Beijing.

But for some reason, the plane was turned around, and flew towards the Indian Ocean instead of China for at least an hour: https://upload.wikimedia.org/wikipedia/commons/f/f2/MH370_fl...

Merely incompetent crew or a technical failure would not cause that - it has to either be a hijacking or an unstable pilot doing it on purpose.