Most of the Founders had fought in the Revolution. They handled the Tories a great deal more harshly than Lincoln handled traitorous CSA sympathizers.
Look, you can’t have it both ways. If the CSA was a foreign nation, as it and its US supporters claimed, then US citizens conspiring and/or acting to aid and comfort a nation at war with the USA were indisputably guilty of treason by the Constitution’s definition.
If the CSA was not an actual foreign nation, it and its supporters were among the “enemies domestic” the Constitution speaks of.
The Constitution is a very stretchy document. The Founders never intended it to stretch over a civil war. That’s why they allowed for civil rights to be suspended in time of emergency.
Traitorous?
Many {Like myself} would like to take part because we want not by force...Many of the States joined the Federal Compact and made it clear that they reserved the right to leave! During the debates{in Madison's writings} He made it clear that if either party violates the Constitution it becomes VOID and of no force... Whom formed whom? We, the delegates of the people of Virginia . . . Do, in the name and behalf of the people of Virginia, declare and make known that the powers granted under the Constitution being derived from the people of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression, and that every power not granted thereby remains with them at their will . . .