EgyptAir Flight 990 (1999)
When this Boeing 767 crashed, it forced engineers to improve cockpit procedures and forced extra pilot training. This entire crash happened because of revenge. The pilot, who was reprimanded and blamed for supposed sexual allegations, felt forced to crash the plane. He killed all 217 passengers and staff on board, including the captain who punished him. Because of this incident, cockpit procedures and background checks were enforced to prevent human emotion from getting in the way of hundreds of lives (The Guardian).