Why Northerners Opted to End Slavery

The United States was about democracy and social equality from inception. In the late 18th century it was nearly alone in that regard. Slavery is the antinomy of freedom and implicitly inimical to democracy. It is a danger to free men.
The world governments of that era were royal with kings and queens, except for the United States. Forming the nation wasn’t easy. It required revolution, National independence came first, perfecting the union of free states need be achieved later.

At the time of the revolution all British colonial entities allowed slavery. During the revolution some states began the practice of ending slavery. After ratification of the U.S. constitution in 1788, many New England States were free states while southern states that regarded African slaves as capital assets were known as slave states

Some historians have the opinion that northerners regarded slavery as a reinforcement to the paradigm of oligarchic and aristocratic political forms. It was viewed as a threat to democracy. Britain continued to be a primary threat to the existence of the United States. After burning the U.S. capital in prosecuting the War of 1812 wasn’t successful in reestablishing British dominion in America outside of Canada England, continued to find the south a weak underbelly to work mischief upon. Northern states had a protectionist trade policy concerning Britain for much of the period. Though most Brits that weren’t members of elites opposed slavery, Britain sold trade goods vital to the south during the war. Liverpool was the main port of export to the slave states. 

http://ldhi.library.cofc.edu/exhibits/show/liverpools-abercromby-square/britain-and-us-civil-war

The free state-slave state duality continued while the nation grew in the first half of the 19th century. Laws passed by the federal government had priority over state laws. Some federal laws inflamed states citizens over a half century concerning slavery issues. The wikipedia relates this time line on select slavery issues, as well as one from England…

“The Fugitive Slave Act of 1793, a law passed by the United States Congress. The Slave Trade Act of 1794, a law passed by the United States Congress. The Slave Trade Act 1807, an Act of Parliament in the United Kingdom. The Act Prohibiting Importation of Slaves, a United States federal law from 1807.”

https://en.wikipedia.org/wiki/Slave_Trade_Act_of_1794

As these laws were implemented the effects were substantial. The fugitive slave act in particular was onerous and offensive to northerners because it seemed to directly contradict their liberty and opinion; it required that escaped southern slaves that were in free states be returned to the southern slavers. An upgraded Slave Act in 1850 is regarded as a proximal cause for the civil war. It was an issue that was critical for liberty concerning not only slavery, but the power of elites and aristocrats over democracy.

Southern states couldn’t tolerate states that harbored escaped slaves, and northerners could not tolerate giving up people to slave states. There was still much Christian opinion in the nation in those days concerning the equality of mankind. Northern Christians regarded Africans as human rather than as capital, while some southern Christian leaders such as General Gordon, used wrong Biblical hermeneutics to support their opinions justifying slave ownership. The south sought secession rather than to have their rights to own and control slaves reduced, and war ensued. The war was fought not simply to end slavery. It was also fought to keep the United States and democracy from falling apart and sinking back into the vast human historical establishment of subjects ruled by rich, powerful elites.