> At what point did the German people find it morally acceptable to start killing Nazis?
Functionally never, after the war the Allies handed "denazification" over to the West Germans to handle themselves in 1946. They then let basically everybody off and lots of former Nazis were let back into power. By 1957, 77% of the West German justice ministry was comprised of former Nazis, which was actually a higher percentage than the amount of Nazis in the ministry during Hitler's reign. The Soviets were more strict with the denazification of East Germany, but many former Nazis were still able to retain high positions in government and society.
If it helps, the nazis themselves probably asked this very question before they started killing ... whatever the contemporary term for nazis was back than.