There's a problem with this argument.
Confederate states actually controlled the federal government prior to their succession, and strongly opposed states' rights to ignore federal laws like the Escaped Slaves act. Wisconsin and Vermont had judges and legislators who declared any individual who reached their borders to be free.
Abraham Lincoln had declared opposition to slavery, but said that he would not impose federal law on the states, and had not even threatened to free slaves before the war began.
So while the Civil War did have a lot to do with states' rights, the Confederates were opposed to them and the Union supported them. This led to the secession, which sort of flipped the script, because then it was Confederate states demanding the right to leave the Union, and the Union saying "no" to that particular demand. It had very little to do with telling Southern States that they could not keep slaves, although there is plenty of reason to expect that it would have gone in that direction without the Civil War. Abolitionists were gaining ground, and Lincoln was morally opposed to it, so it the Confederate states had a reasonable expectation that they would eventually be forced to end slavery, but it hadn't happened before the start of the war.
To be sure, the Confederacy was a traitorous band of anti-American bigots seeking to create a tyrannical ethnostate of christofascists (fuck, why does that sound familiar?). However, in this case, the answer to Velma's question in this case is not "keep slaves" but is instead "secede."