Apparently our moral high horse only extended to those people who could be financial threats to the crony capitalist power structure still running the Capitol today.
If it was about slavery, once slavery was abolished the country would let the states go and become independent countries. They had a right to secession. If fighting slavery required suppressing secession, once slavery ended they wouldn’t try to stop states from seceding.
Slavery is a fig leaf. What they wanted was total economic control, and that's what they got.
Had the Southern states been of no economic value, and no economic threat, they would have let them go without a fight.