Here is part 1 of the Stanford Prison experiment in the 60s. Remember that these are merely volunteer students who have been divided into A team (guards) and B team (prisoners) and then the results were recorded without intervention...until it got too serious and it had to be halted.
This is an extreme example but it is instructive non the less.
http://www.youtube.com/watch?v=2o0Nx31yicY
Edited to fix link.