To conduct killings on such a massive scale, Nazi Germany had the help of many people, including ordinary citizens and foreign governments. Most places the Nazis invaded found willing participants to assist in genocide. For a general overview of perpetrators of the Holocaust, see Perpetrating the Holocaust by Paul R. Bartop and Eve E. Grimm.
How much did ordinary Germans know about the Holocaust? Robert Gellately argues in Backing Hitler that not only did average citizens know and condone Nazi actions, they often aided the Nazis by reporting suspicions and denouncing enemies of the Reich. In Perpetrators, Victims, Bystanders, Raul Hilberg offers a thorough examination of those who participated in the Holocaust and those who stood by silently and watched, both in and outside of Germany.
The majority of the perpetrators faced no punishment for their murderous actions, while many of the surviving victims suffered long after the war. Additionally, few showed any remorse or guilt for their actions. Donald McKale’s Nazis after Hitler offers further analysis of this topic.