The proper treatment of slaves is in the bible....you may want to rethink that.
Slavery can be a humane and correct element of a society, but only in a monarchy, where every one is a servant of the crown. In such a society slavery is just the most extreme form of servitude. And slavery can only be humane in the case where the monarch is the proverbial Philosopher/King.
In the United States of America, founded as a representative democracy, where all people are endowed with life, liberty and the pursuit of happiness there is no room for slavery.