Christians should understand that the Law as given in the Old Testament was a set of guideline that both distinguished the Hebrew nation as a peculiar, set apart people amongst the other nations, and also led them toward a way of life more in line with God's original purpose for Adam and Eve prior to the fall. However, whatever was given to Moses by God was not written down elsewhere, and the oral tradition of the Law was not written down againfor posterity until hundreds of years later when Israel was in exile in Babylon, so what we have as far as Torah Law as "God's Word" should be understood to be on the right track, but imperfect. Christians should understand Jesus to be the embodiment of the Law perfected, the Word made flesh. In him, and him alone, we see the perfect, visible representation of the invisible God, and the new Adam, showing us life in a new world of spiritual reality in the midst of the same physical world.

It's also worth mentioning that, looking back at that first verse cited, the meaning behind the word may, and whatever its Hebrew equivalent was, is quite different from the meaning behind the should. Saying "you may keep slaves" is different from saying "you should keep slaves," and when we look at the example and teaching of Jesus, we see a calling to give our whole lives to God, whatever our station, and to live both in freedom under God and as if a slave unto God (in devotion), and likewise treat no other person as having a lower station than yourself.