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.
Except that, as you've repeatedly said, states have the power to sever their relationship with the United States for any reason at all, so a state's desire to kill all left-handers is just as valid legally, if not morally, as any if one follows your line of reasoning.