First off, your ideas about North and South are far too clear cut. Slavery had at some point existed in all the colonies, but in most areas of the north, slavery was simply not very profitable or economically useful. Slaves were used when a particular commodity required hard labor and many hands. Most New England industries and small farms didn't qualify. New York had a sizable slave labor force working in shipyards, but a 1741 slave revolt helped to push New Yorkers toward removing slavery, not so much for "Slavery is wrong" but because "it's safer this way." In the South, however the major commodities were tobacco, rice, and later, cotton, all of which required large land tracts, a large labor force, and was labor intensive to maintain. Plus cotton only grows in certain climates suitable to the South.

Back more to your question, by the 1840s many Southerners defended slavery for multiple reasons, to include economic and racial reasons. But there was a key difference between being anti-slavery and an abolitionist. Both believe slavery is wrong, but being against it for different reasons. Abolitionists, who rose to prominence in the late 1820s, believed slavery to be a moral evil, and needed to end immediately everywhere. Even by 1860 they represented a minority in the US, concentrated in New England mainly. Even Lincoln was not an abolitionist by most standards. But it is true that much of the North was antislavery. For some, slavery kept the white men down by not allowing for competition in jobs. Others felt slavery was inconsistent with republican principles. Others felt it was morally wrong to hold slaves, but felt it must be ended gradually and freed slaves should be returned to Africa (see Liberia and the American Colonization Society). What is important to understand is that oftentimes being against slavery is equated with "not racist." This just simply isnt true for most (at least white) Americans in North and South. Thousands in the North did not support slavery for moral or economic reasons, but plenty equally wished to keep some degree of white supremacy. (For a further look at this, research the second Klan from 1915-1930, the Great Migration, and the immigration laws passed in the early 1920s)

Let me give a different perspective.

First, it's a mistake to look at slavery as a static institution. Slavery in 1600s America was far different from what it became by the mid-1800s in the South.

Second, it's also a mistake to think there was ever some epiphany moment. From the very start, when slaves were first brought to America in the 1650s, there were white people all over the country lobbying against it.

So rewind a bit. When slaves first were brought to America in the mid-1600s, Europe was still kind of in the final stages of the Middle Ages and a lot of the white people who came over to America came over as indentured servants. You signed some contract and were indebted to a landlord for some term, usually 4, 7, or 10 years. The landlord would provide the land and some animals, and then whatever revenue (not profits) you made, then you had to split them 50/50 with the landlord. But you also owed a minimum amount, so if there was a drought one year and the crops failed, you were pretty screwed because your family would likely have to live on a starvation diet for the winter and/or you were going to have to extend the term of your contract to make up for the amount of crops you were contracted to deliver.

There were a bunch of other ways that lords screwed over the servants--like, if they were the first servant to live on the land, then they were expected to build a house and barn. But when the contract was over, the house and barn belonged to the landlord, who could sell and profit from them or give them to the next indentured servant.

Anyway, slavery at the beginning wasn't entirely removed from this condition. You usually didn't run your own farm like a servant did, nor would you receive any profits from your work from your slave master. But usually, in the 1600s and early 1700s, slavery was not a lifetime condition. It was a condition that lasted for a fixed term or a fixed amount of money. After 10 or 18 years, the slave was supposed to be granted their freedom. Or, they had to pay their lord back $500 or however much it had cost to buy the them, which essentially, would translate into being s lave for a decade or decades.

Slaves could earn their freedom because back in the 1600s and early 1700s, slaves in America had certain rights. The specifics varied from colony to colony and even town to town, but generally, in the Thirteen Colonies before the French and Indian War, as a slave, when you weren't "on the clock" working for your master, you were free to go earn money from other people in town or in any other way you could, and you were entitled to keep that money. Slaves were entitled to go to church, and they were entitled to assemble. They were allowed to get married and even baptized in church. A slave's spouse or children could not be sold away from them--families had to be kept together. And a slave master could not beat you or abuse you and if they did, then a slave had the right to sue their master in court. Or anyone else, for that matter, if someone had broken the law against them just like any white American.

So slavery at the start was not nearly what the Roots version of American slavery portrays it as. Slaves weren't brought off the boat and immediately beaten wantonly. Their rights weren't entirely trampled on. They did still have some rights, even if behind the scenes, there were some nefarious slave owners who didn't obey what the law said they had to do.

Yet, just as soon as there were slaves, there were people protesting against slaves. The Quakers, particularly, were an early anti-slavery force, petitioning against it in Germantown, Philadelphia, in 1688, just a few years after Pennsylvania had first become a colony and pro-slave forces were first introducing the practice to the area.

And despite the relative amount of rights that American slaves in the 1600s and the first few decades of the 1700s enjoyed, it was still a terrible situation to be in. And as time went on, it became more and more obvious that slave owners were continually employing legal tricks to keep a slave a slave. If a slave needed to earn $500 to pay for his freedom, the slave owner would always somehow find a way to prevent him from getting that much money together. That kind of thing. The same thing often happened to indentured servants, too--both slaves and servants were money in the bank and slave owners hated to have to lose all that free income they were getting.

Occasionally, one local event or another would flair up local tensions and violence would break out, such as the New York Slave Revolt of 1712. These early revolts often saw black slaves and white servants team up to take on local landlords.

So, what was the reaction to this? In New England and Pennsylvania, these early revolts were enough to prevent the institution from taking off. There were still only around 600 slaves in Massachusetts at the time, and even fewer in Pennsylvania and most of the other New England states. But in the 1720-50s, pro-slave forces started to try to move in to these areas and were successful to some degree, but politically, there were immediately met with abolitionist forces. But then the French and Indian War got in the way in 1753 and pre-occupied the political atmosphere in America, and was almost immediately followed by the American Revolution. All the while, the abolitionist forces in New England and Pennsylvania were becoming a runaway majority that as soon as the Revolution was over, Massachusetts made slavery illegal by court order without much fuss. The other New England states and Pennsylvania followed in short order--as soon as they were no longer under the control of the British government, they had all outlawed slavery before 1800, either immediately or over time.

New York and New Jersey were a little bit different because of their Dutch history, which was a little bit more pro-slavery. But after the early riots and rebellions in New York in the first half of the 1700s, public sentiment began cementing against the institution so that they, too, both began phasing it out in the 1790s, thought not abolishing it completely until the 1820s.

In the South, the story started out the same, but after rebellions in the early 1700s down there, such as the Stono Rebellion of 1739 in South Carolina, the reaction of the local governments was entirely different. Whereas in New York, it had been taken as a sign that the practice needed to be got rid of, in the South, it was taken as a sign that they should take more rights away from the slaves.

So it was then and only then that the Roots version of slavery started to arise. Little by little, the Southern states took away the slaves' rights. They could no longer assemble--it was too risky that they might plan another revolt. As a result, they often couldn't go to church, or even get married. Slaves had no standing in court, so a slave master could beat you and you could no longer sue. A children and spouse could be sold separately from their family. And most importantly, it was during this period that the facade of slavery being for only a fixed term went away, and now slaves could be owned in perpetuity. And for the first time, a child of a slave was also a slave. This had not necessarily been true before the early- to mid-1700s.

This is how the split between North and South really began to emerge. They had both started out with this institution called "slavery" that was somewhat less severe in the beginning. But as slaves revolted, the North decided for moral, religious, and economic reasons to abandon it early on, while in the South, it developed into a harsher and harsher institution.

And as this happened, Northerners began to get increasingly upset and the national abolition movement began. Because then the other thing that started to happen is that slave laws of Southern states and anti-slave laws in Northern states began to butt up against each other, as slaves ran away or Northern and Southern citizens interacted with each other and the laws of the states conflicted.

Anyway, that's the gust of it. It wasn't that suddenly, Northerners had this epiphany about slavery. The TL;DR version is that slavery had started out entirely differently back in the 1600s, and during the 1700s, the North and South took diverging paths, where the North began to grant black people most of their rights while in the South they were taking what few rights black people had away from them. This drove them further apart, and an abolitionist movement arose, followed by some legal hostility, followed by a Civil War.