Undoubtedly Khufu's subjects would be aghast if you shrugged at damming the Nile; perhaps the Romans would be aghast if you shrugged at oral sex, conscientious objectors, and female bosses instructing male employees; Hindutva activists today are aghast if you shrug at eating hamburgers, to the point that they may lynch you if they catch you slaughtering a cow. Some of our moral values are of course the correct ones, slavery being actually objectively immoral unlike those other examples, but generally people only know their culture's value system, not the objectively correct one. And some of our moral values are definitely not the correct ones.