Execute them on the spot for attacking US interests.
They didn't attack US interests. They formed a separate nation which the North invaded.
Well, no. They attempted to steal US property.
No, they didn't. With the exception of a few forts, the US government owned nothing in the Confederacy.
And one of those forts was filled full of American troops when the rebel forces started firing upon it.
The OP is full of BS, but let us not forget- Federal troops didn't fire first.
And not one of those soldiers was killed, and they were allowed to return home. Lincoln, of course, knew the fort would be fired on when he tried to resupply it, and that's exactly what he wanted. Public sentiment was against forcing the south to remain in the Union, and Lincoln needed an inciting event to change that. That's not to excuse the south for firing on Fort Sumter, of course, because that was obviously dumb.
Regardless, I don't understand the idea of calling these people traitors or wanting them to have been executed. Who cares if they wanted to leave the Union? How would you be worse off today if the southern states had a separate government? Frankly, I think most northern Democrats would enjoy getting their way more often if that were the case. Of course, I can see why Lincoln didn't want them to leave. He wanted them to pay their taxes and he didn't want his protectionist Union to have to compete with a free trade Confederacy, but I don't get the modern sentiment that the south and north are better off together. The political rancor I see seems to suggest otherwise.