I am reminded of Milgram's experiment. Although that experiment ties into authority (whereas this case doesn't), it showed that 65% of the people in his sample were willing to use a lethal electric shock on someone they could not see, but that they could hear screaming, because the doctor was strongly coercing them to continue and said he would assume responsibility if they died. Humans are certainly capable of terrible things.

Also see: http://en.wikipedia.org/wiki/Stanford_prison_experiment