"I am not, nor ever have been, in favor of bringing about in any way the social and political equality of the white and black races... while they do remain together there must be the position of superior and inferior, and I as much as any other man am in favor of having the superior position assigned to the white race." - Abraham Lincoln, 4th Lincoln-Douglas Debate
Again, I'm not going to deny that Slavery is the Souths's "great black mark" upon its history. But claiming like the North had the absolute moral high ground in the Civil War is simply bunk.
By and large, other than a generalized sense of distaste, the North could not have cared less about Slavery, or Blacks. They went to war simply to hold onto what they perceived as being "their land" (regardless of whether the spirit of the American Revolution, or the people actually living on that land, agreed or not), and out of a macho sense of bravado which lead them to believe they could easily put "backwards" uppity Southerners in their place. They got the absolute piss slapped out of them - by forces a fraction of their size, no less - for their arrogance. Quite frankly, they kind of deserved it.
That lead them to step back and re-evaluate, and come back to the conflict with a cause that was a bit more morally worthwhile.