In the north you had a minority who were pro-slavery, probably a majority to whom it was an issue they probably had opinions about but not opinions that consumed them (kind of like the problem with AIDS in Africa or global warming would be to most Americans today- they may have opinions and concerns but ultimately they’ve got enough problems in their own life to give it a whole lot of concern), and then you had the anti-slavery factions who were a large minority.
The anti-slavery faction was mostly divided into two very unequal parts. The vast majority were anti-expansionists. Anti expansionists were the more pragmatic; they were against slavery and probably would have agreed “It’d be wonderful if we could click our heels together and have slavery be illegalized and everybody fine with it” but they knew this wasn’t going to happen without a war, and it wasn’t worth a war to them, so they were content to contain it.
The abolitionists were the more zealous: slavery is an injustice, a barbaric practice, and it must be abolished and if that means war so be it. They were of the “Let justice be done if the heavens fall” camp. They were a small minority of the whole population.
Lincoln was anti-expansionist. He had been against slavery for ethical and moral reasons since he was a young man, but he did not want to fight a war over it and felt the country had enough problems to deal with even without abolishing it and was fine with southerners keeping their slaves so long as they kept them in the south and didn’t try to take them into the territories. Lincoln, left to his own judgment, would probably NEVER have tried to encourage any type of abolitionist agenda through Congress and probably wouldn’t have signed it if it passed, until the war of course.
The South was also very diverse, but the simple answer- one that is easily verified a thousand different places, is “They seceded because of slavery”. They had many other reasons to be very against the North- some of them very legitimate- but none would have led to war.
You’ll read that “Most southerners didn’t own slaves” and that’s true, but it’s also true that the South was a plutocracy and most of their policy makers DID own slaves, and while not all southerners owned slaves themselves at least a good quarter to half of them (depending on what part of the south they lived in) were from families that owned them, and the ones who didn’t own them certainly didn’t want to compete with between 4-5 million freed slaves for land and jobs. The founders of the Confederacy made absolutely no pretense that they were seceding for any reasons not directly related to keeping slaves, and it’s not opinion when I say that the claims “It was state’s rights, not slaves” is latter day nonsense that came long after the war.
The Federal government saw Southern secession and the firing on Ft. Sumter and all as a rebellion- i.e. they never recognized the legitimacy of the Confederacy to exist or the authority of its government. Imagine if today all of the states west of the Rocky Mountains banded together and said “We’re now the Western States of America, a new country”, and they seized all government buildings and military bases by force: the government now would do what they did then and say “Oh hell no” and send troops in to deal with the rebellion.
So the North’s reaction was not to free slaves, it was to end a rebellion and basically throw out the [admittedly so large it’s hard to quite define it] domestic terrorist regime that had taken over some of their richest lands. There were certainly some abolitionists who said “Well if we’re going to war anyway let’s go ahead and burn this mother called slavery down in the process”, but they were a slim minority. Later, for military reasons and because many people who prevsiously would never have fought a war to end slavery basically said “F*ck it, we’re in a war anyway and we’re never going to have a better opportunity” they added the eradication of slavery to the objectives.
It was not a universally popular decisions: many troops were outraged at the notion of using the war to free slaves. Most just wanted the war to be over. Lincoln’s decision was part ethical, part pragmatic, and largely “Well, the tornado tore down the house and we have to rebuild it, so might as well paint it a color we like better than the old one”.