"The real question is what makes a Nazi a Nazi?"
In my opinion:
Exterminationism padded with moral-sounding arguments to make it look like justice.
The Nazis gave plenty of reasons why the extermination of the Jews was justified. The Marxists ("Progressives") do it in a more laid-back style: "Ya know, I'm not saying suicide bombing is justified, BUT you can't occupy and oppress a people for more than 60 [sic] years and expect nothing to happen!"
You just get the idea they'd greet an Iranian atomic attack on Israel (God forbid) with crocodile tears: "I grieve for the people who were destroyed because they were so uncompromising and addicted to their ill-gotten gains of land."
As for the fact that such an attack could be the end of the Sudeten-Arabs as well as those "Zionist oppressors"—well, you can't make an omelet without breaking eggs. So long as the Sudeten-Arabs exist, there's something to blame the Jews, uh, I mean Zionists for. Not Naher El Bared, their massacre in Lebanon by fellow Arabs, because that doesn't serve the purpose.
The Nahr al-Bared refugee camp in Lebanon was subject to a massacre in 2007.
But as this was a case of Arabs killing Arabs, no one in the west cared. Certainly the progressive-left never cares when Arabs kill Arabs or when Muslims kill Muslims or when Muslims kill Jews or Christians.
This is because the progressive-left has given up entirely on notions of universal human rights and thus basically stands for nothing.