Yeah it’s one of those things where, like, the Union was initially fighting to stop secession, not stop slavery - but they very much cared about slavery. They wanted to bring the South back into a national government wherein slavery’s time was numbered, at best, and were not willing to compromise on that.

And, y’know, fuckin’ rightly so. Sixty years of compromise had led to no progress, so fuck putting it off for another sixty years of human beings being raped and tortured for the wealth of a few wealthy white bigots just to have a civil war crisis again anyway. Most Northerners - even most abolitionist Northerners - would’ve been deeply racist by modern standards, but awareness of the evils of slavery was too much even for them. Anti-slavery sentiment was genuine, strong, and widespread, even if stronger in a “We can’t let it come into any other state” than in a “We have to stamp it out in the South” sense.