If your ancestor lost their citizenship during the Third Reich and you apply for naturalization by descent under art. 116 Basic Law, you do not have to prove your impunity.
However, you must meet certain requirements in order to apply for naturalization under the Nationality Act. The general rules are set out in Section 12a of the Nationality Act. In the case of naturalization on the basis of descent, however, the requirements for a clean criminal record are less stringent. Persons who have been convicted of intentional crimes and sentenced to imprisonment or juvenile detention of two years or more, or who have been subject to a preventive detention order at the time of their last final conviction, cannot acquire German citizenship under Sections 5 and 15 of the Nationality Act.