Slavery has existed and will continue to exist as long as humans are on Earth. Every culture on every continent practiced slavery. In the early 1800's 3/4 of all humans on Earth lived in some sort of involuntary servitude. Slavery was no more or less evil prior to 1865 than it is today. Like most things involving humans, slavery was an economic issue. Owning slaves was a necessity for operating a large agricultural venture, slave owners would have gladly given up their slaves if an alternative existed for cultivating the crops the economy depended on. The yeoman farmers of the Northeast and Midwest did not have large scale farms that required large workforces so slavery made no economic sense in that region. The farmer and his family could tend to the land and the crops. The Southern plantation owner did not have that option and there weren't enough people to hire in the rural South to supply the workforce needed. Slavery was the legal, logical answer to the labor needs of the South.
British slavery ended in 1838 so it isn't like the US was some long term holdout on the institution. It should be added that British slavery didn't end until slave owners were compensated by the govt for the value of their lost slaves, another example of slavery being a function of economics and not any sort of intentional "evil".
If the Southern states had not seceded, slavery in the US would have likely continued merrily on until mechanical agricultural equipment was invented that would have made the need for slaves unnecessary. At that point, slave owners would have gladly freed their slaves since mechanical equipment would not have required housing, clothing, feeding and medical care 24/7/365.