“The depravity of man is at once the most empirically verifiable reality but at the same time the most intellectually resisted fact.” - Malcolm Muggeridge
Good and bad are not absolutes, they are relative to their culture. Were our slave-owning founding fathers good or evil? Evil by today's standards but good by those of their time. Apples and oranges.
Yes and no.
As you point out, during the time of slavery the average consensus was that slavery was not ideal, but it was OK. After decades of being make illegal, however, the notion of slavery is absurdly evil.
The same can be said of abortion. Before Roe vs. Wade, the consensus was that abortion was not moral, but decades after Roe vs. Wade abortion is viewed as not ideal, but OK.
As you can see, people are nothing more than sheep to be herded and told by the state what is right or wrong, just as the Bible states.
The key to both, however, is dehumanizing the victims. In the case of slavery, men were viewed as glorified apes to justify the mistreatment of them, for you see, we all have an internal morality to treat others as we would want to be treated. The only way to then get around this internal policeman is to devalue the entity you wish to oppress, such as calling an unborn child a "fetus".
History is full of such examples, whether it be Nazi's calling Jews vermin or Muslims calling unbelievers infidels.
Dehumanization is the first step towards oppression and genocide.