The idiocy never ends with you. You leave out the actual "slavery" thing to present an ignorant and specious analogy. Americans object to people being held against their will for no valid reason.
I have no interest in discussing a condition that would have been maintained in the Union had secession never occurred. I find it disgusting when people try to force the Civil War into the straw man of slavery. I consider this a cowardly dodge by people who don't want to address the actual causes of the war.
Slavery was going to continue being legal when the Union launched it's war against the South. The North had absolutely no intention of changing slavery so long as they kept getting the money from it.
This is why I consider all these moral claims to be so much crap. Nobody in the Union invaded the South for a moral reason, they did it for money and raw power. As Charles Dickens said, the slaves would get tossed into the bargain, but could just as well have been tossed out of it.
"Every reasonable creature may know, if willing, that the North hates the Negro, and until it was convenient to make a pretense that sympathy with him was the cause of the War, it hated the Abolitionists and derided them up hill and down dale. For the rest, there's not a pins difference between the two parties. They will both rant and lie and fight until they come to a compromise; and the slave may be thrown into that compromise or thrown out, just as it happens."
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Americans object to people being held against their will for no valid reason.
If there had been any intentions of doing anything about this when they invaded the South, you would have a valid moral point, but since the intent was to keep them in their chains and working in those fields that were making so much money for New York and Washington DC, the Army sent to kill the people of the South doesn't get to use the slavery argument to justify what they did.
They had no intention of doing it when they began.
The war has to be justified under the purpose of the invading force when it was began, not for some reason made up nearly two years after the start of the war.