you would like to think all crew members are above suspicion but I remember vaugely, an incident that I think involved a DC10 freigher .... a company employee (flight engineer) on a free ride, attacked the flying crew with a hammer with the intention of taking command and flying the aircraft into the companies headquaters ...... fortunately he was over powered but caused desperate injuries to both pilots ..... I think I'm right in saying that one of them never flew again ...... I remember feeling so sad that a fellow "Airman" could do such a thing .....