Not for treason. While the union was on the right side of the slavery issue, I can't agree with the whole imperialist "you can't leave this nation, we rule you" stance. Governance should be consensual IMO, though I'm not opposed to military interventions to deal with evil shit like slavery or refusing to allow those who want out to leave (but not to enforce economic advantages).
Though ultimately it doesn't really matter and the strongest militaries, intelligence agencies, and civil obedience forces will decide in the end, as they always have.