There was probably a hope that Hitler would destroy communism. As for the stories of human atrocities, there were similar stories during World War I, but after the war it was realized that, while there was some, not nearly as much as people thought and rumors said. There was no real right and wrong side of WWI. So when stories about the Holocaust were being told, too many people thought and said "It is probably just like the stories we heard in WWI which turned out to be mostly wrong." However, after the war we horribly came to the understanding that this time the stories were not wrong and that WWII was very different from WWI.