The simple answer to the "mystery" is a scenario about a "lone wolf" with the determination and the capability to direct the plane to the middle of the ocean and die with the plane.
We saw a confirmed example of such a guy almost exactly one year later (8 March 2014, 24 March 2015):
We saw a confirmed example of such a guy almost exactly one year later (8 March 2014, 24 March 2015):
https://en.wikipedia.org/wiki/Germanwings_Flight_9525
The rest are just the details.