Jesus says God is like a slave master.

Luke 12:46-48

The lord of that servant will come in a day when he looketh not for him, and at an hour when he is not aware, and will cut him in sunder, and will appoint him his portion with the unbelievers.
And that servant, which knew his lord’s will, and prepared not himself, neither did according to his will, shall be beaten with many stripes.
But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.

I have heard the argument from some Christians that “slavery” in biblical times was different and that this somehow excuses the lack of condemnation of slavery within the bible. However, I was interested to find what the Quest Study Bible (New International Version) had to say about the scripture quoted above:

Why would a master cut his servant to pieces?

In ancient times, those who owned slaves held complete mastery over them. Slaves were considered as property and one who displeased his master could be severely punished, even killed. Such deeds were within the master’s legal rights. Jesus may have used this particularly gruesome punishment to illustrate the spiritual truth of his example: that a hypocritical leader will be cut off from any life with God.

The question remains: If god of the bible is omnibenevolent, why doesn’t the bible condemn the owning of a person as property (slavery)? Slavery is probably the most brutal and dehumanizing institution every created by mankind. Why isn’t the condemning of it a commandment? Which is a greater act of immorality: Coveting something the neighbor owns or owning another human being? Working on the Sabbath or owning someone as property? Graven images or a human not having any freedom their whole life?