"Any deed that any human being has ever committed, however horrible, is possible for any of us—under the right circumstances."
The infamous "Stanford Prison Experiment" was aborted in less than a week due to its character-imploding power. Along with Stanley Milgram's studies of obedience to authority—the "shock experiments"— the investigation is considered one of the most important pieces of research demonstrating a core tenet of social psychology: external situations can lead us to behave in ways that we would not, could not, predict.
American Scientist Online features details in Robert Levine's review of "The Lucifer Effect: Understanding How Good People Turn Evil," by Philip Zimbardo
Also, Discover Magazine Online has this excerpt