It's all psychological inertia. For example even if they knew that Communism would cause tens of millions of deaths they would not nuke USSR.

We can't know.

As an extreme example, antisemitism was so commonly palatable that an open admirer of Hitler, Charles Lindbergh, was a very serious candidate for the presidency.

Today, open antisemitism can get one's career derailed in much of the world. Would that be true without the eye-opening shock that the news of the concentration camps produced?

IN NOW WAY was the Holocaust forgivable, but we simply cannot say if the world today would be "more moral on average" or "less moral on average". Maybe Jews would feel and be far less safe today in predominantly Christian countries. Maybe not.

All we can say for certain is that the horrors enacted upon the Holocaust victims were evil. As we can surely say that murdering many millions of people living in the USSR would be.