Attack #1 against Slavery
“All instances of slavery abuses people and treats people as less than human, therefore slavery is immoral.”
Wrong American slavery, and slavery practiced outside of Israel may have treated slaves as less than human and it was therefore immoral. But in Israel slaves were guaranteed certain human rights that God commanded.
Attack #2 against Slavery
“Even if Israel treated their slaves more kindly they still were treated as less than human because they did not have equal rights and were not free. All adult humans must have equal rights including full autonomy.”
Wrong God is the one who grants our rights and while he has guaranteed certain human rights to all he did not guarantee an equal amount of rights to all. It is not immoral, or treating someone as less than human to give some people more rights than others if we are following God’s Law in doing so.