Why did hans junior accuse his father of cowardice? what was the "mistake" papa hubermann had committed?

During Hitlers 40th birthday a book burning was prepared in Molching of books written between World War 1 to rise of the Nazis. The  Hubermann children Hans Junior a soldier and fanatic Nazi accused his father of being disloyal to Germany and asked why Liesel was not reading Hitler's book Mein Kampf. Hun then called his father a coward for doing nothing "a whole nation cleans out the garbage and keeps itself great''. He stormed out and Death explain how Hans Junior ended up at the battle of Stalingrad where Death was carrying the souls of dead soldiers.

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