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I hate to be the history nitpick but the whole indiscriminate killing of Jews in incomprehensible ways was so well hidden that most of the Allies didn't become aware until they were so deep into Germany they stumbled across Auschwitz. If I recall correctly, it was the Soviets who juggled with their own morality to take the German officers as prisoners or just execute them on their knees in the mud.
Most of Germany itself didn't even know the extent of the Holocaust until after the war.
western allies were informed quite openly in late 1942 and quite possibly earlier, they just didn't believe it at first for some reason https://en.wikipedia.org/wiki/Raczyński's_Note https://en.wikipedia.org/wiki/Witold's_Report
Didn't believe or didn't care? Anti-semitism was hardly unpopular back then.
The reason being that the war never was about attrocities but about sphere of influence?