While it is true that the United States of America, and the British colonies before that, practiced what Abraham Lincoln called one of the twin relics of barbarism, slavery (the other one being polygamy), it was not the only nation in the world to do so. In fact, all other nations of the world practiced some form of slavery, and the United States was the first nation in the world to outlaw the practice, having fought a bloody war against itself to do so.
One of the myths of slavery is that the American South benefitted economically from the practice. The theory goes that with free labor the South was able to prosper on the backs of the enslaved. But the labor of slaves was not free to the landowners, they incurred expense in housing, feeding, clothing, and providing medical care to the slaves in their possession. Whether they paid a work force that then used that money to provide all those things for themselves, or in the case of slavery, the landowners provided those necessities for their work force, it was still an expense.
Slavery did not make the South more prosperous, in fact, the free North had a much more thriving and diverse economy than did the South. And had the Civil War not ended the barbaric practice of slavery, the free market would have. As manufacturing grew to replace agriculture as the mainstay of the American economy, and rural life was replaced with urbanization, the impracticality of slavery would have rendered it useless.
Many people, especially on the Left, in deriding the Constitution argue that since many of the Founding Fathers had slaves, that fact somehow nullifies our founding. The Founding Fathers in the South were born into a culture which had included slavery for over a hundred and fifty years before they came into existence. So while they detested the practice, they were trapped in the maze of slavery they did not create. But they very wisely devised a Constitution that they knew could be used by men in a subsequent time to insure the undoing of slavery. Had they pushed for an end to slavery as part of the founding of their new country, there never would have been a union of the colonies that became a beacon of freedom and hope, not only to former slaves in this country, but to the millions of enslaved and oppressed throughout the world.
Many of the physical abuses suffered by American slaves were exaggerated for political effect. I am not saying that no slaves were ever abused, only that it made no sense for plantation owners to pay good money just to abuse them. It is analogous to someone today buying a car and then beating it with a baseball bat. It could happen, I suppose, but it is not likely in most cases. In fact in the case of James Madison and Thomas Jefferson, their estates were managed by trusted slaves the years they were absent during the revolution and the early days of the new government.
I would not want anything I have written here to misconstrue that I somehow support the idea of slavery, or think it was anything but a blemish on this great nation's past. But I also think what is purposely lost in the teaching of slave history in the United States is the fact that it was the goodness of our founding which eventually lead to slavery's eradication.