You need an ideology that glorifies death for this sort of thing to be possible, as people aren't going to do it out of pure tactics, and you'll need the ideology to be popular if it isn't going to be counterproductive. What you need, then, is a Resistance that is more like the Fascists. Essentially this requires a very far-back PoD that has WWII a stomping match between different types of nutter rather a war between democracy and totalitarianism.
Expect they were doing so in relative secrecy, only incredibly well-informed people knew what was going on or what Generalplan Ost really entailed, and of them only a few morally principled people cared. Hitler's speech about the invasion of the USSR doesn't mention Living Space once, neither does his speech announcing the defeat of Poland. Nazi Propaganda maintained that the each move the Nazis took was nothing to do with conquest or extermination, but an act of self-defence from a 'ludicrous state' that was committing genocide against Volk German communities (Hitler's justification for attacking Poland) or a Soviet that was poised to attack Germany at any moment and was ravaging 'brave Finland.'
Many people, too many people in fact, would simply dismiss such stories as allied propaganda. Hell, the US dismissed such stories because they sounded like bad propaganda. It was only when troops marched into the camps that people really faced facts.