Apparently there is no consistent moral argument at work here; its an argument of expediency to justify an outcome of which they approve.
Now you are going to make them mad. With facts like these, you puncture their moral superiority balloon. The claim that the Union fought the war to abolish slavery is the only fig leaf they have to justify what horrible bloodshed they caused.
In the one case the moral imperative of ending slavery justifies a war against the rebels and the need to exterminate all traces of their culture 150 years later; in the other case it never even gets mentioned. Go figure. I guess slavery only became interesting to the North sometime after 1783.
It only became significant to the Union in 1863. For two years into the war, they were going to let in continue, and indeed, did let it continue in their own states.
Still pushing that lie I see.