Secession was not over slavery but what the Federal Government was doing. It is easy to come to that conclusion because it was an issue even before the states ratified the Constitution in 1786. It had more to do with the perceived power grab by the Federalists against the Statists. Had Lincoln not one, a plan was ready to be taken to overturn at the last 30 years of laws Congress, who was majority made of the Northerners or those that leaned in the direction of the North.
Think about this, if the Southern democrats had done what the modern democrats had done today, then the Civil War would only have been postponed.
Again the Civil War was over States’ Rights not slavery. The industry of slavery was the major industry of the time. It was an after thought to the Civil War, meaning it didn’t even occur to politicians until at least a year and a half into the war to free the slaves. You go to the original source documents, especially Lincoln’s thoughts on the matter. Lincoln says it himself the war was not about slavery and it was a by product of the war not the reason for it.
State's right to do what?
"The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution African slavery as it exists amongst us the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution." - Alexander Stephens
That's from March 1861. Before the rebellion began.