Maybe the co-pilot saw the smoke and tried to put it out and the pilot would let him and there was a struggle as he tried to take control. That explains the 90 degrees left and then the 60 degrees right before it plunged into the sea.
or perhaps there was a switched fire extinguisher on the flight deck. Co-Pilot operates it and instead of fire retardant it issues an aerosol poison. All on flight desk pass out; plane goes into a frenzy and then crashes.