And adding to that: People saw what happened to their Jewish/Romani/Disabled/Communist/Queer neighbors. They worked in the clinics where disabled children were selected and sterilized or (sent off to be) killed, knew the police men who rounded up homeless people etc. And people came back from concentration camps as well. Some people did time there for ordinary crimes. German citizens incarcerated in concentration camps even had to pay for social security while incarcerated, so that their insurance history wasn’t broken afterwards.