There are a lot of Holocaust books written from the point of the view of the victims. But what about the ordinary people that turned into killers? How does this transition occur? How do they justify their actions? What do they feel as they go through these steps? In Ordinary Men: Reserve Police Battalion 101 and the Final Solution in Poland Christopher Browning documents the story of a particular group of Germans who went from ordinary citizens to cold-blooded killers.
This particular group was chosen because of the degree of documentation available. Personnel transactions, interviews, trial transcripts etc were all available to be analyzed, allowing the author to paint a picture to a degree of detail that would be difficult in most situations of war.
There are a lot of stats in it, though. I found the first half pretty dry as the author throws a ton of numbers and descriptions at you so that you have the context to understand the scale of what happened. For me, this is the type of concept that is better illustrated as a novel.
Nevertheless, if you need a reference book on this topic ;) , I highly recommend it!