In a way I think we're being tempted to indulge in sick fantasies and think it's OK because it's happening to Nazi's.I think I should have left out the "in a way". That's how the film was (misleadingly) marketed.
The idea of righteously slaughtering enemies does seem to appeal to us. Lot's of fiction panders to this. It always bothered me in Lord of the Rings the way killing orcs is treated. It's one thing to kill them in war when they're the aggressors, but orc slaughter is often treated as a good thing in itself even when a particular orc isn't threatening. Humans are encouraged not to show mercy or take prisoners because of how mean the orcs are. We're okay with it because they really are nasty pieces of work, they're literally created that way! specifically so we don't feel bad about enjoying their suffering. It's not like individual orcs have a choice in anything, what if some of them were sensitive poets, what the hell are they supposed to do? Defect to the humans? Yeah right.
Orcs are not just bloodthirsty and hate filled, they're ugly and smell bad. Even though they're pretty intelligent, we're encouraged to think of them as lower than animals and that it's a moral duty to exterminated them. A service done for the world and the future. Given what we know about how humans treat outsiders and how genocides usually proceed in real life, is it wrong of me to find this a bit creepy?
(I know it's "just fantasy" and I am a big LOTR fan. But I do think it's true that films like this are appealing to a dark side of our nature.)