Simplify the following text for a Grade 6 native English-speaking person.
Original: Kellner was Laubach 's chief justice inspector during WWII and wrote a 10-volume diary about the misdeeds of the Nazis , later published as a book , My Opposition .
Simplified: Kellner was the chief justice inspector in Laubach during WWII . He wrote a secret diary about the misdeeds of the Nazis .