IMDB describes the movie thus:
The rise of national socialism in Germany should not be regarded as a conspiracy of madmen. Millions of "good" people found themselves in a society spiralling into terrible chaos. A film about then, which illuminates the terrors of now.But the central character is not that good, and he only gets sucked in when the madmen have already taken power. So I don't really buy this as an account of what the film is about.
He gets involved with the Nazis because of a novel he writes about euthanasia. The Nazis like his sympathetic stance and recruit him to do philosophical and propaganda work, first on euthanasia and later on "the Jewish question." I was expecting more about euthanasia, but apart from seeing a room full of people with Down's syndrome (who are presumably to be killed), it doesn't come up much in the later parts of the story.
Along with the craziness there was a kind of rationality about the Nazis: none of this old-fashioned nonsense about marriage and fidelity, let's just pump out more (Aryan) babies; if someone's too old or handicapped to be useful, or perhaps even happy, put them out of their misery; if you want money and there's money in Poland, take Poland; and so on. If you combine a reading of Nietzsche as saying that Judaism is the root of (almost) all evil, i.e. Christianity, "the one great immortal blemish of mankind," with a physicalist reductionism about beliefs then you might almost have the makings of a consequentialist argument for the Holocaust. You would have to kill the Christians too, probably, but it's not as if there is no method discernible in this madness. (Not that unthinking prejudice was not a huge part of it too.) And roughly this kind of rationality, used to different ends, is familiar enough in consequentialism, capitalism, and probably socialism too. (Wittgenstein seems to have thought so: "The spirit of this civilization makes itself manifest in the industry, architecture and music of our time, in its fascism and socialism, and it is alien and uncongenial to the author."
Disappointingly, the movie does not explore this idea.