Slavery is with us to this very day, so by what standard do you judge it wrong? As to judging that which we read in the Bible, remember that it is history, written by men informed by the Holy Spirit, but still reading events by the imperfect lights of their own narure and culture. If God’s own people could not recognize the Lord when he came to them, and they did murder him. If the Apostles, men who were willing and able to got to their deaths to spread the Truth were often uncertain of the right thing to DO, and did not always do the right thing, Then that means we are still in ther midst of history and must still work out our salvation in fear and trembling, because perfect clarity will not come until the Resurection. Regarding slavery, Paul’s little message to Philemon says it all. The small seed that Paul planted took many years to grow to the point where we understand what our duty to our fellow men is. Even among the barbarians of Germany, perhaps more so there than in the midst of a civilization with five thousand years of institutionalized slavery, it did not eradicated inequality, but it muted it, by adopting the principal that no baptized person shall be a slave like the slaves of antiquity.
Black slavery was a departure from this principle, and it was this principle that lead to our Civil War. The long-time justification that slavery was inevitable because of the sinfulness of man was not, however, entirely false. Sin leads to injustice, and where we kill one unjust institution , another springs up to replace it. This will not come to an end until both master and the slave have the same heart.