I wonder how many people today, who condemn slavery as evil, would have had slaves back then and not thought anything of it (and I believe it would equally apply if you reversed the colors). I also wonder what people in a future century will condemn us for doing right now...
I just don't think that you can compare slavery to what the Nazis did: the scientific, industrial disposal of people. Slavery had been going on for thousands of years.
And still is.