You do know Capitalism was making the slave trade unfeasable. Technology is superior at mass production. It increasingly made sense to invest in machinery etc. The most skilled slaves were eventually given free reign and only had to pay a fee to their master.
White nations and their Capitalist technology ended slavery - at least in the Western world.
It isn't capitalism that ended slavery, but humanism.
Capitalism, no matter how technologically advanced it is, will require employees to maintain all those machines. And what kind of employees cost the least, thus making the best profits? Slaves.
The American civil war was capitalism vs capitalism, the only difference was that one side was more humanist than the other.
It is only a thriving (free market) society that has the resources and time to spend on humane projects.
A free market society that embraces technology requires less and less manual workers. It does require more skilled (hence free) workers - not slaves.
See farming.
The American war of succession - where one side humanely denied a divorce. Forced relationships are so humane!
One side wanted to keep slaves, the other didn't. Guess which one was humanist. And which one was capitalist (hint : answer may be both).
The question isn't if the war was rightful or not, is why one was for slavery and the other wasn't. Both being capitalist, this probably isn't the answer.
My opinion goes toward humanism. Maybe wrong, I'm not a specialist in the history of slavery, but I doubt you are either.