I don't see how you can completely separate racism from the 30's onward from before that. Again, the people forced to give up slavery resented the forces that made them do so, as well as the former slaves themselves. You acknowledge this now, but are not looking at the full ramifications of what this would mean (i.e. how it would play out over time).If this is true then why was the actions of the North just as racist (in terms of policy)?
What I am saying is not that slavery was a good thing, or that racism did not exist, but that the issue is much more complex than simple and superficial treatment will allow.
Many in the South did view black people as human beings. Many as equal in worth as themselves. Even many slave owners spoke out against the "evils of slavery" (which I always found a bit odd). And in the North people were equally racist but their racism manifested itself differently.
But yes, there is a lot to be said about Southern states resenting other states forcing change. That is true and that probably contributed a lot to resistance.
That said, it is not right to take racism today back to slavery. It is dishonest. The reason is that (as we discussed) the issue goes back to black men selling other black men for profit. So the institution of slavery in the US is only half way back. But more important, it goes back TOO FAR. The issue is not what happened over a century ago. The issue is what from the 1930's forward.
You may not realize it but the Great Depression was a leveling factor. Those who make assumptions based on slavery use blinders. In all fairness, the black community today has benefited from slavery (based on the fact that life in America, even as bad as it apparently is here, is a better standard of living even for the poor than is tribal Africa). That does not mean slavery is a good thing. But it does mean that God uses all things for His glory and people need to realize the larger picture.
Today's racist ideas may have been instilled by a culture of resisting imposed change, but I do not think so. I believe this is human nature.
They immediately tried to replace slavery with things like debtor's prisons (where they had many right back on the plantation) and other forms of discrimination, and even violence. Blacks had managed to make gains as great as becoming governors (fulfilling General Benning's dread!), but when the North pulled out, and their influence was removed, this was quickly halted! The KKK arose, terrorizing not only blacks, but also those sympathetic to them. They created the stereotypes of the blacks being dangerous violent criminals and racists to justify all this (from Benning, again: "The consequence will be that our men will be all exterminated or expelled to wander as vagabonds over a hostile Earth, and as for our women, their fate will be too horrible to contemplate even in fancy"), and produced propaganda like Birth of a Nation promoting this and the KKK.
That now is into the 20th Century. This just continued through the 30's, and resentment grew then and beyond, as they were forced to make more and more concessions on race.
You said you always found a bit odd that some spoke out against the "evils of slavery". Well, some people knew it was wrong, but societal pressure still led them to go along with it. If people in high places like Benning believed as they did, then when slavery ended, they did not just drop those beliefs and fears (and the whole "black" issue), only to be suddenly picked up again in the 30's.
And since people say "God uses all things for His glory and people need to realize the larger picture." regarding slavery, then why don't they see it that when when it comes to changes they don't like. So God "used" slavery (to save them from their tribalism), but taking down monuments (that offend people) is the work of the Devil! Why?
(Though I always agreed with the last point, that racism was man elevating himself over another not to downgrade the other but to elevate himself. Still, the "net effect" is the same).