Our Union -is- voluntary.
When the people grow tired of it, they can exercise their constitutional right to amend it, or their revolutionary right to overthrow it.
The people of the states may disunite without being killed, by holding state conventions and voting to secede?
You mean like New Yorkers reserved the right to do when they ratified the Constitution:
That all Power is originally vested in and consequently derived from the People, and that Government is instituted by them for their common Interest Protection and Security.
That the enjoyment of Life, Liberty and the pursuit of Happiness are essential rights which every Government ought to respect and preserve.
That the Powers of Government may be reassumed by the People, whensoever it shall become necessary to their Happiness that every Power, Jurisdiction and right, which is not by the said Constitution clearly delegated to the Congress of the United States, or the departments of the Government thereof, remains to the People of the several States [emphasis added for Walt], or to their respective State Governments to whom they may have granted the same; And that those Clauses in the said Constitution, which declare, that Congress shall not have or exercise certain Powers, do not imply that Congress is entitled to any Powers not given by the said Constitution; but such Clauses are to be construed either as exceptions to certain specified Powers, or as inserted merely for greater Caution.