That is the question that comes to my mind again as I ponder the scenes described in recent news accounts. There are no simple answers but there are clues from extensive research done not only on the Nazis but also on other perpetrators of genocide. Genocide is the attempt to destroy or eliminate a people or ethnic group. Most genocides are undertaken on a smaller scale, involving an effort to drive a people group out of a region or to destroy them within that region or nation. That's what happened in Kosovo.
Genocide is a political act.
There is no recorded instance of genocide that has not been either orchestrated or at least permitted by political authorities. With genocide, a government gives its blessing on the use of force against noncombatant civilians of an ethnic group perceived as an enemy. Sometimes that use of force is directly ordered. So the troops on the ground are actually carrying out the orders of their commanding officers, who are carrying out their orders, etc. It is difficult, though certainly not impossible, for various links in that chain of command to resist the evil deeds they are asked to perform.