There's a really interesting book called, "Ordinary Men" about a reserve police battalion (basically, middleclass professional guys who never expected to be called up or see serious violence) that carried out war crimes. One thing that's worth noting is that the men in the battalion are much more willing to carry out atrocities if their friends are there. The author argues that it's not 'fear of punishment should you refuse' that motivates them. It's 'fear that by refusing, you're being weak and putting the responsibility for carrying out an atrocity on your peers.' All these guys knew that what they were doing was horrible, but they bought into the fiction that it was necessary and inevitable.