Harrowing is probably the only word to describe it. I don't think the context-placing introduction is necessary. It would be totally fine if he, like many other authors, plunged us up to the ears in his world without explaining what the world is. Other authors do this all the time. I think it might have made it slightly more effective.
Though I certainly can't question anyone who's lived through the horrors of war, the horror does seem a little excessive. Almost like those high school shows where “something” has to happen in every episode, this is horror upon horror upon horror. Could someone, a child in particular, really survive all of it? It seems to me that the violence is slightly gratuitious, though I do fully understand why it is so violent.
On the other hand, I totally agree with the morals: it wasn't just a few bad apples in Germany; evil is banal and most of us will not resist it if the majority aren't resisting; evil begets evil.