Hitler hated Austria. He couldn't make into art school there. He hated Austrians and for the most part, Austrians hated him. That's why he left.
Doesn't change the fact he was Austrian and Austria willingly participated in his enterprise. Arnold's father wasn't conscripted into the Nazi party or the SA. Arnold is aware of the history and those who participated.
His father was not found guilty of any crime but he belonged to an organization that sanctioned such crimes.