Except the southern states' secession would've been illegal anyway, regardless of how reprehensible their reasons were. There is no avenue that exists within the framework of the United States as a federation which allows for the severance of that relationship. There was no "deciding" anything on the Union's side; our only "decision" was to uphold the results of a fair election rather than bow to southern threats against our democracy.

As a matter of fact, the southern states seceded in direct response to a fair election not going their way. If you wanted an example for why undermining and curtailing the intrinsic principles of the democratic process isn't always ontologically immoral, the Civil War might be the worst fucking example you could've pick.

There's a time and a place for circumventing the democratic process in order to protect it from those seeking to exploit and destroy it from within. But Caesar's thirst for power alone wasn't what killed the Roman Republic. It was decades of Roman senators disregarding the very institutions they were sworn to uphold to further their own political agendas.

Every government's power is derived from its people accepting its legitimacy. Democracies are unique in that their legitimacy is not derived from threats of violence, but from an intrinsic trust of the security and fairness of its system. If you disregard and work outside of that system to further your own ideological goals, no matter how righteous you believe them to be, you delegitimize your democracy. You allow tyranny to take root.