So yes, there may be a Constitutional case for a state's right to secede from the Union, but the Southern states did not secede just to show they could, to demonstrate the proof of the abstract principle of states rights. They seceded because they felt their pecific right to slavery was in danger from Lincoln and the North, and then used the argument of states rights as a justification. I really don't see how anyone can plausibly deny the primary role that slavery played in the decision of the Southern states to secede.Now suppose you don't consider blacks to be human beings. That's unacceptable today in the US, but was not at the time of the Civil War. Under that assumption, the real issue becomes a dispute over property rights. And are property rights not worth fighting for? It does no good to project the mores of the present onto the past.
I have ancestors who fought on the Union side, and I have spent a lot of time in the South. I have no particular irrational attachment to either side. But I sure hear a lot of sanctimonious BS about the Northern cause. It's no surprise - history is written by the winners, and you'd better have a hell of a case to justify 600,000+ casualties.
Kindly note that the Civil War started in 1861. Lincoln did not sign the Emancipation Proclamation until 1863, and it only freed the slaves in the states in rebellion. The fact is that freeing slaves wasn't all that popular an idea in the North either, and Northerners weren't particularly enlightened on race issues - integrating Boston schools a century later was like pulling teeth. Lincoln himself favored shipping blacks back to Africa - here is a story of his encounter with Sojourner Truth.
Did Lincoln have any reasonable alternatives? Perhaps. Suppose he had tried to see things more from a Southern point of view. He would have noted that ending slavery would have had grossly disproportionate impact on the South. South Carolina was the first state to secede, in no small part because emancipating blacks would have left whites in the minority there. That's above and beyond the gross dislocation in the economy caused by emancipation.
You might say "it was evil - it had to end, case closed, damn the costs". OK, did it have to cost 600,000 casualties and the rest of the legacy? For that you could have spent a lot of money compensating slaveholders for their losses and still come out ahead.
And who should pay? I have to wonder just how many of the rabid abolitionists were in the front lines - it was well noted that the war was a rich man's war and a poor man's fight (see this too).
Don't tell me there's no economic calculation or else we'd be invading several African countries about now.