But it's impossible to separate those issues. If I understand you correctly, then announcing that I'm now a foreign country because I want to, say, kill all the left-handers, is just as fine a reason as any, and opposing by force such an announcement is somehow more evil than breaking up the United States in order to murder left-handers.
If "Left-Handers" are deprived of rights under the existing Union, then the Union has no moral high ground from whence to condemn whatever happens to "Left-Handers" when someone else does the same thing they did.
This argument is pushed even further into the realm of absurdity when the Union argues that "We'll let you keep killing left handers just so long as you rejoin the Union."
It shows that their priority is control, not creating new rights for others.