Even if another pilot was present, from that altitude a suicidal co-pilot could easily cause an unrecovable dive and crash. If two pilots pull/push controls to opposite directions they kind of neutralize each other, and obviously the suicidal one would have element of surprise as an advantage.

In fact just that happened on EgyptAir flight 990. A suicidal copilot was able to crash the plane even with another pilot present, and that happened from a higher altitude.