The end of the film takes us to the present day, where various caretakers maintain Auschwitz, now preserved as a museum, not a museum of Nazism, but a museum of the atrocities committed by the Nazis.
In real life, Rudolf Hoess was hanged after the Nuremberg trials. Although nothing in the ending contradicts this, he is shown with blood from his mouth, an incurable death cough and stress-induced vomiting, suggesting that he developed some kind of disease or allergic reaction to the camp's pollutants, and the film cuts to black. Rudolf's bout of nausea at the end of the film is a reference to the ending of the documentary The Act of Killing, in which a retired genocidal soldier is hit by a wave of vomiting when he experiences remorse.
It's unclear if the museum was Rudolph's vision or if the film makers just sort of transported us into the future, but what is certain is that Rudolph's descent into darkness is symbolism, a hint of where the man is actually headed - into darkness.