In The Book thief, how does Zusak demonstrate the contradictory nature of humans?

I think this alludes to the paradox that man can be both good and evil. Throughout the book we see people like Liesel unquestioningly put lives on the line for other people. They hide Max in their basement knowing full well the horrid punishment coming to them should he be found out. Hans Huberman gives a starving Jew a piece of bread in the open and is sent away for it. At the same time some German civilians and the Nazis say and do unspeakable things to the Jews in the book.