The natural way for society to progress is to have honest, candid discussion where all parties can make their point in an environment of good faith; where anytime someone finds themselves in the wrong, they feel free to admit it without being cast out.
Slavery is wrong; it was wrong when the Egyptians did it, it was wrong when the Romans did it, it was wrong when the Muslims did it, it was wrong when the Spanish did it, it was wrong when the United States did it, it is wrong that the CCP does it. Because a wrong thing was done does not condemn all things that were done to being evil or wrong. Most of our current moral values are derived from tradition.