Well, the Nazis have a great defense. They can suddenly ask "What happened in Katyn Forest in 1940?". During this incident the Soviets massacred thousands of Polish civilians. The Nazis can also ask, "What happened in the Ukrainians in 1936-1939?". The point is that the Nazis will point out that the Allies backed Stalin, a man responsible for genocidal acts as well.
Second, the Nazis will suddenly ask, "Why are you concerned only about the Jewish people?". They will point out that starting in 1933 until 1939, the Japanese massacred the Chinese people with complete impunity (e.g. Marco Polo Bridge Incident, 1936). While millions more people were massacred by the Japanese military, the Allies seem to be obssesed with protecting a small minority group. When the Italians massacred Ethiopians in 1935, did the international community do anything? When the Indians were massacred by British troops in the 1920s, did the international community heap scorn?
Third, they will point out that many of the ideas that led to the "Final Solution" actually came from the Allies. Consider the legal justification for ethnic sterilization was based on Buck v. Bell (1928). The idea of the concentration camps was adapted from the 1870s policies towards the Native-Americans. The Protocols of the Elders of Zion were written by the head of the Russian Okhrana. Henry Ford and Charles Lindbergh were supporters of the Third Reich, throughout the 1930s and 1940s.