Greetings in that Divine and Most Precious Name of Our Lord and Savior Jesus Christ!
Philemon owned a slave too. It was a different time with a different culture.
Who knows? In 200 years people may look at us and say how barbaric and evil we were for killing animals for food, or for allowing 18 year old children to have sex legally.....
You also noticed how Paul was being sarcastic to Philemon when he chided him about owning their fellow brother Onesimus, when he said things like "no longer as a slave but more than a slave, a beloved brother, especially to me but how much more to you" and further "if then you count me as a partner, receive him as you would me." Did Philemon treat Paul as a slave? I think the guilt-trip aspect of this Scripture should be fairly obvious.
For well over a thousand years, the Ethiopian Orthodox Church was complicit with the East African slave trade. Ethiopian Christians participated in trade networks, collected head taxes on slaves as if they were just inventory, and sometimes even joined in slaving raids into the south and west of Ethiopia. Further, many monasteries in fact owned multitudes of slaves. It was not until 1930 that HIM Haile Selassie finally by God's Grace liberated the slaves of Ethiopia. He abolished to paternalistic model which had dominated the Ethiopian Christian mind, very similar in fact to the way the Spanish treated the American Indians, and instead raised the bar as to how the Christian civilization of Ethiopia was expected in a nationalistic way to embrace the idea of a pan-Ethiopian society. HIM understood the inherit wrongness of slavery, and no amount of Scriptural apologetics could have changed HIM mind, or indeed shifted the tide of the world which shifted away from direct slavery to economic exploitation (just as America shifted from slavery to the insidious share-cropping system).
We as Christians can pretend that God justified slavery by the Bible, in all actuality, much like all the other evils of the world, God allowed slavery to exist out of His mercy for the slave-dealers, slavers, and slave-runners. However, from the beginning of time, God created us a family, and only Cain's jealousy of his brother caused man to fight against man. Slavery is the worst form of evil, because it entirely disregards the human dignity of the person enslaved.
For those who pretend the Old Testament some how exonerates post-Christian slavery, perhaps you should begin to contemplate stoning witches and adulterers just the same.