Ephesians 6:9. And, ye masters, do the same things unto them, forbearing threatening: knowing that your Master also is in heaven; neither is there respect of persons with him.
God never condoned slavery. He never condoned a lot of things the human race has done. However, He does understand us, and He knows what we are capable of. So He set some parameters around our behaviors, and this is one of them.
Don’t mistreat your slaves. Remember, God is the Master of both of you, and in His eyes you will stand on level ground with those you mistreated. You aren’t special. You do not have the right to mistreat anyone, not even your slaves.
I think maybe this verse needs to become our national motto. Not that we have slaves any more, and haven’t for a very long time. But our mistreatment of each other is ungodly, unrighteous, and unjustifiable. It is for God to settle accounts with unrighteousness, and He will.
We need to watch our steps.