Tonight I thought: what if you were the son and your father told you he did it? Or that he and his friends did it?
In real time, that would make you 60 and your father 80, probably (or maybe older; again: no research). But in fiction, you could change it to when the father was 40 and the son was 20; or the father was 50 or 60 and the son was 25.
Whatever age it is, it could make for an incredible story. Think of it this way: You love someone, deeply, all your life, and then they tell you they did this horrible thing - and part of it would be that he's not apologetic. I don't know exactly how to build that in, but it makes it harder to reckon with.
Okay, trying to flesh it out, it gets difficult. How is the father a three-dimensional character without being blatantly racist all the time (or obviously racist)? And would it be possible for someone to be "less" racist when they were older and still have done this thing? If he's the kind of person who no longer spouts the racist stuff, how is he not apologetic for it?
If the son is horrified by it, how does that work? If he's not racist and his dad is, how does he have a good relationship with him? Because the book could start with the revelation and then we would slowly learn the father is racist after learning a few facts about what a caring man he was from the son. But then how is the son not racist and okay with his dad, or have fond memories of being raised by him? O would his father's racism not have affected him, one way or another?
It's complex, and I'm tired, and thats why I don't read books. But I think it's a fascinating premise and it could be played out in many different ways (and maybe outside of race, it would be easier; I made it about Emmet Till because the mystery is intriguing). The Nazi war criminal would be the simplest (and perhaps the weakest, being so obvious; but that would depend on who wrote it).