Normal men like you and me. I don't believe in innate evil. He had a loving wife, children who adored him. He was a good family man, did not beat his wife, was not an alcoholic, did not have even a parking ticket to his name. And he was no exception. All the top leadership, the Allegmeine SS officers, the Doctors, etc were all good decent respected men with no taint - other than the fact that they were effectively carrying out the World's biggest organized massacre.
And yes, they slept well, discussed politics, religion, showed mercy(except to the untermenschen) and did almost everything we do. Yup, we are capable of great evil, given the proper circumstances.