Chance of pilot suicide is very low. Airplane always have co-pilot. And i don't think another pilot want to die. Any sight of nose dive a hundred people airplane co-pilot will have trained to quickly prevent that?2 things.
Either pilot suicide,
Or complete and utter failure of all airplane's critical systems
There is no other explanation for how the plane would descent so suddenly. Whatever the reason, the investigators will have a tough job and I am not even sure if the black box remained intact after such an impact