The Lucifer effect describes the point in time when an ordinary, normal person first crosses the boundary between good and evil to engage in an evil action. This term was coined by the psychologist Phillip Zimbardo who conducted the famous Stanford Prison experiments in which normal people were grouped as 'prisoners' or 'guards'.
Within a short amount of time the guards became extremely aggressive towards the prisoners, so much that the experiment had to be shortened due to the severity of their treatment. The Lucifer effect describes when a situation turns normal 'good' people into perpetuators of immoral or 'evil' behaviors. The name comes from the mythological story that Lucifer was a good angel who became evil and was banished for his actions.