Oh, horsespit.
We've known from the days of Moses that slavery is evil.
The Southrons who declared independence rather than put up with northerners who disapproved of them were acting out of a desire to destroy criticism of a system they all knew in their hearts was totally Satanic. And they paid the price for their folly.
No need to refight the Civil War. We won, you lost, get over it.
Northerners were heavily invested in the rum and slave trade in the 18th Century. Slavery was legal in the northern colonies until 1774, when RI became the first to free its slaves. Slavery continued in DE and MD until 1865.
Northerners were the first to consider secession, at a convention in Hartford, CT, in late 1814, to protest the already ending War of 1812 and the decline in the fortunes of the Federalist Party.
Secession was considered constitutional until the force of arms stamped it out.
Lincoln's notion that the union preceded the Constitution is is without historical foundation, but it has been accepted to justify the war. My understanding is that the South paid 2/3 of the tariff receipts prior to 1860, and that was Lincoln's principal motivation to prevent secession.