Yes slavery did exist, but it existed on both sides. Abolitionists were limited to Boston plutocrats, whose fortunes were probably founded in part on the slave trade, German anababtists, and Quakers. The abolitionist fire-brands reigned a steady stream of condemnation from their pulpits, but provided no feasible alternative to the practice, or realistic transfer of the cash-poor, seasonal economy of the South to the wage system of the industrialized north. And oh yes, the factories of the North, manned by subsistence wage-earners, do not appear substantially superior to the conditions of the African slaves, especially when you consider that agriculltural work was regulated by a growing season.
It's to Lincoln's credit that he did understand those difficulties and attempted to put some in practice; the firebrands undermined him.
Claiming the war was about slavery perpetuates a lie, and demonizes a hell of a lot of good-- I mean very good-- people. Libeling the south does not make the north look any better. But the main problem is that it overlooks the certain result of chopping this nation apart in the face of Europe's imperial ambitions.