Pilot suicide? If the pilot put it into a dive, we'd see a big increase in speed as the altitude decreased.
Stall? Spin? If that were the case, we'd see the speed getting dangerously low before the altitude drop and even then, it should be recoverable from that altitude. That drop in altitude would give more than enough speed to recover from a stall.
Instead the altitude and speed both suddenly dropped at the same time, which seems to indicate a sudden structural failure of the aircraft.