I'd take an entirely different approach. I'd make it a point to show the contradiction in the cultural relativism's argument if he were to favor one culture over another, especially when said culture was exterminating others. Also point out the danger of Godwin's Law.
Perhaps point out some of the bad arguments and then destroy them with the above. Find view, find contradiction in view, apply. Then of course point out that an invasion is not necessarily an extermination in the case of the allies going against the axis, whereas what they were doing was....
This is actually the type of question that is dangerous on its own without guidance and the crazier among us could take literally and go off the deep end. I have taught my students to examine views of incredibly unpopular leaders and their methods, then to see if they have any bad points and what those bad points are. Also there may be some good methods hidden in a given bad leader's ideas. In the case of the Nazis, all I've been able to come up with is good tasting strudel and good rocket scientists like Von Braun (who we poached after the war), otherwise meh, not so good as an understatement....
Its about identifying and decoding bullshit._________________________________________________________________________
Also I have a job interview and I'm freaking out a bit. Meh.