Only after the Christians started the concept that man was created with certain god given rights, which only originally applied to certain groups then was extended to all, did it start to be questioned whether slavery was immoral or not. So, yes, as society evolves so does morality. We condemn the history of slavery through the prism of our current morality. No one in the Roman Empire would have said slavery was immoral, no one in ancient Egypt would have said that enslaving the Hebrews was immoral, and only a few northern Protestants thought slavery immoral at the time of the Armerican Revolution. There are parts of the world today where slavery is still practiced and it is not thought immoral in those cultures.