Liesel waits on the steps of a church watching the book-burning die down to ash, until Hans arrives and asks what's wrong. Liesel has been deducing that the Führer is the source of all her problems. She asks Hans plainly if her mother was a Communist, and if Hitler took her away. Hans cannot deny it, and then Liesel states that she hates Hitler.
For Liesel (and later Max) Hitler becomes the personal face of all the injustices of war and hatred. Liesel connects the dots and realizes how she feels – this hatred for Hitler is naïve now, but it will become a deeper force later, inspiring her small acts of revenge.
Hans wants to hug her for this, but instead he slaps Liesel in the face and makes her promise to never say such a thing again, especially in public. Then he makes her practice her "Heil Hitlers" for a while. Liesel feels terrible, but her time for thievery has almost arrived.
A moment defining the important difference between inner life and public life – Liesel can hate Hitler in private, but she must love him in public or else risk danger to herself and her family. Hans realizes the danger in her words, and must slap Liesel to make her understand as well.