As you might recall, the Stanford Prison Experiment pitted several student against each other as randomly chosen prison guards or prisoners. The end result was quite gruesome. Guards enforced authoritarian measures and even inflicted psychological torture on the prisoners. The experiment is a classic note in psychology texts and has often been used to explain the Nazis, Abu Ghraib, and other forms of inhumane practices.
Apparently, there are serious flaws with the study. This post at the British Psychological Society details some of the issues: