In my view, America's original sin was the European settlement, which was a crime against the native peoples. They treated them wrongly again and again, although some tried to find a way to get along at the beginning.
Slavery was the second sin, but didn't really appear in the English colonies until around 1640 or so. Oddly enough, it was a free black man who owned the first black slave.
Kelly was right that the war wasn't just about slavery. Both Lee and Lincoln prioritised the union over slavery.
Kelly didn't say anything in favour of slavery. Folks are putting those words in his mouth. He was talking about a process to avoid the civil war, which killed a load of Americans. Parliamentary compromise worked elsewhere. It was a failure of American law and government that it didn't occur in the US. Look at your constitution to see the villain. It empowers intransigence.
And look at your Supreme Court: it could have referenced Lord Mansfield and demanded positive law to justify slavery, and without it said it must not be.
He was right that Lee chose his state because his country was at war with itself.
He was right that decent people owned slaves. Washington and Jefferson owned slaves and were aware of the issue. And yet they kept them all the same. I can't see any reason to suppose confederate folks in the 1860s were worse than the Founding Fathers for doing the same thing.
He was right that applying today's morality to the past is the artifice of shallow people. No genuine historian does that.
It's only single issue fanatics that think judgements are made about single things. But sometimes there are several issues involved in a decision.
The folks jumping down Kelly's throat are the knee-jerks in my view.