If a book is good, I like it. And if I knew about the problematic content, and bought and read it anyway, I don't think I get to complain later. To me, that's like buying and wearing a fur coat knowing what atrocities are behind it, and then giving the boutique a poor review because, cruelty.
I also don't think a moral lecture belongs in a book review - or a book, for that matter. I often wanna read a good ole Coelho or something he was involved in, and suddenly I find myself jesused.
If the reader has known about the problem before buying/reading, there's no grounds for complaints here I think, and any deduction of stars should be for other issues. Not for taking offense in offense you signed up for.
Now, is anything ever off-limits, ethically? It should be, perhaps, but it's never gonna happen. Controversial or not, even Nazis get to be in love, and so do Jews, with the wrong people. While I'm no fan of the "jesus me out of this" thing either, it makes sense as a safety measure. I haven't read the book and don't know how strongly it advertizes religion, but even if it may seem questionable, if it works for the story or for the character, it should be used.
If a book is beautifully written, if it's well researched, but if the central idea is problematic for the genre: a high-ranking SS officer willfully complicit in genocide as the hero... and a Jewish woman who converts...
Do you review it as it stands, for what it is outside the greater issues, or stop and say "You know, this isn't okay."
If I know what awaits me, I consent to the content. If it's not okay with me, I can simply not read it.
As for the double standard - complicit in genocide, yet protective of one beloved Jew - this is human nature at its finest. We are all hypocrites. We pet our dog while eating a cow's baby, we screech about injustice within our own circle yet leave poor people down the street to starve, we want to be loved but make so many feel unloved. I think a Nazi and a Jew are a marvelous pair to illustrate this hypocrisy and dilemma of moral convenience. As a symbol, they should be rubbed in our faces over and over and over.