Other articles say it was taken for testing.
Probably some sort of drug/poison.
He probably locked the cockpit door, set up the AP for the descent, and then took something to knock him out.
He probably could not have gone through with it, sitting there watching the Alps get bigger and bigger.
Nope, an aviation expert said he had to turn off the AP before it automatically pulled up.
A couple of later threads say it was evidence of his conversion and attending the radical mosque in Bremen.