Our acts are evil when we harm individuals for the sake of strengthening our place in the group. Harming individuals is known or felt to be wrong even in childhood. Passion is required to defeat that knowledge. Evil acts are done in passion, against self awareness. But there is a sort of institutional memory that is produced when the society we live in rewards repetition of the same evil acts. The repetition itself becomes the symbol of evil. Individuals then have to work hard to forget, to avoid reading those symbols. They use bureaucratic language, in which people are "items" and "units". They define people as things, to be treated in accordance with technical rules. They find laughable, or enraging, that what are really things pretend to be something they are not, something like themselves. They use drugs or other intoxicants to ease the transition to the passionate self forgetting the evil act requires.
The special evil of the Holocaust is this: it produced self aware knowledge of the evil that was being done, and taught the means of forgetting that knowledge. Memory and forgetting, one after the other. The Holocaust showed that the human species has learned how to turn itself into a perpetual motion machine for evil.