As terrible as this is, it has all the ingredients of a James Bond or Mission Impossible movie set up.
Who or what was on board that some nefarious power wanted eliminated? What leverage did that nefarious power have on the co-pilot to make him do this?
The simple explanation of "the guy wanted to commit suicide & take everyone with him" seems rather unconvincing. If that were so, why not crash into a populated area to maximize the damage? If it were an act of jihad, where is the call of glory to Allah & death to the infidels? (He knows the voice recorder is going, surely he would send a message of victory.)
I would expect/hope that people who fly airplanes go through some sort of psyche profile & background check. Hopefully something as common as losing a wife/girlfriend or being outed as homosexual or experiencing financial hardship would not be enough to push someone cleared to fly to mass murder-suicide.
We may never know.