Bare in mind here that I am not taking sides, nor am I comdeaming either philosophy. But as philosophies, both Unitarianism and Calvanism crossed many denominational line becoming parts of the religious beliefs of many different Churches. Their teaching effect many of our political discussions even today. Religious philosophies are most often the keystone to a culture.
Jeff Daniels playing the charactor of General Joshua Chamberlain in the Movie "Gettysburg" had a discussion with the 20th Maine's crusty old Sgt. Major (I can't remember his name). Anyway Chamberlain was talking about this Unitarian "devine spark' in all mankind that man was some form of an Angel. To which the Sargeant replied, I'm paraphrasing here, "Yes, but if he is an Angel, he is a "Killer Angel".
That is the point here is that the Philosophical difference between the Unitarian belief that all men are brothers, that they are holy, formed at least the Abolishionist viewpoint. While Calvanism with its belief that while men were created of God they are not "Holy". And that with its doctrine of predestination, that God being an all knowing God gives certain peoples special callings for his purposes, established the Southern culture.
Under the Calvanist philosophy the southern slave owner would have thought that he was doing God will because God had given him the special ability to buy slaves for example, and thereby remove the slave from the pit that he was in. I know that this is idealistic, but man often does justify his actions with Idealism.
The southern planter was not involved in the practise of going to Africa and capturing the slaves, but what he could offer the slave once he arrived in America was far better than what the slave was experiencing in the slave pits and holds of the traders slave ships.
Now this does not justify slavery at all. Was there indeed a market for the work that the slave could provide? Yes, there very much was a monitary element to all of this slave trade. The same as there was a market in the north for low wage workers in factories. Was the monitary element the greates factor in the southern Slave institution there is no doubt about that. But, that was some of the justification for owning slaves was that you were at least temporiarily bettering the Slaves lot. Why Christians in the south saw no general religious problem with owning slaves.