It should not be a surprise that people who thought that the buying and selling of human beings and that profiting from a racialized caste system are legitimate enterprises also believed that attempting to subjugate the entire nation is a natural entitlement.
The Southern states weren't fighting for their right to have slavery. They already had slavery. The North was respecting their right to make that determination for themselves. The North was respecting the Southern states' rights.
It should not be a surprise should it turn out that people who think that subjugating half their populations and criminalizing bodily autonomy also feel they have the right to put the rest of the country to the sword.
The people who do not think it's wrong to reach into other people's bodies do not think it's wrong to reach into other people's states.
Go read this thought-provoking essay: https://siderea.dreamwidth.org/1766175.html