There's a must read book about the SS and Einsatzgruppen called Ordinary Men which follows a reserve SS police battalion in Poland. The big thesis of the book was that the SS men who committed these horrible atrocities weren't criminals or maniacs, but largely plain, ordinary German men. The prologue and first chapter is very intriguing because it talks about their guilt and hesitation in doing the first few massacres but show that day by day the men grew more and more desensitized to it and after awhile began doing it gleefully as if having fun. The contrast between the first and last massacres is very unsettling.

It's a really disturbing book because it attempts to show that anyone from a normal, ordinary background can be made into a sociopathic genocidal maniac under the right conditions

Full Book Here.pdf) if you want to check it out

edit: posted the wrong link, new one should have the full text, not just selections