If a pilot wanted to crash a large plane into something, he would most likely need a weapon to keep control of the cockpit. The co-pilot and nav/engineer at the least would notice something amiss from the course deviations, and would have to be dealt with. There may be rules about minimum number of crew in the cockpit at all times, to pull off a kamikaze the other crew would have to be killed, subdued, or forced out of the cockpit. It might piss off the crew, but it's probably for the best.