The Stanford Prison Experiment
Aliens, monsters, demons... we really don't need to call upon fictional creatures to fill our nightmares. Some of the most frightening fiends imaginable already inhabit this planet: us. Psychological studies have lent some insight into what can cause human beings to perform horrific acts, and the ease with which a "normal" person can turn sadistic given the right circumstances is unnerving.
Take, for instance, the Stanford Prison Experiment, conducted in 1971. Test subjects were divided into two groups - prisoners and guards - and made to roleplay a standard prison scenario for two weeks. After only six days, the experiment was discontinued abruptly when a grad student in psychology came to conduct interviews and, based on what she had learned, objected to the experiment on moral grounds.
You see, the participants adapted to their roles too well. After only one day, some had already internalized their roles, feeling as though they actually were prisoners or prison guards. After a Day 2 prison revolt, the guards became increasingly sadistic in their treatment of the prisoners - they would issue physical and psychological punishment, force prisoners to live with their own excrement, remove mattress privileges, strip them naked... and worst of all, the lead researcher, psychology professor Philip Zimbardo, became so absorbed in his role of prison superintendent that he permitted these actions. The guard's behavior became increasingly cruel, and by the end, it was reported that one-third had exhibited genuinely sadistic tendencies and that most of the guards were upset that the experiment was cut short.
Scary what a little power can do to a person.