The 10th amendment cannot be stretched to cover the treason of the South. It was lies, stupidity and stupid lies. See James Madison’s interpretation of the inviolability of the Union.
The right to secede and revolt is not a lawful right. Therefore, it must be exercised on the battlefield, and either suppressed or realized there. But there is a right to secede and/or revolt, not codified in the Constitution, but inherent in the Declaration of Independence and the social contract between the government and the governed.
In the case of the South, I don’t think it was justified at first. Lincoln might have proved a fair and just President to the South, too. But it became justified after the fact by Sherman’s burning and destroying as he marched towards the sea. After the war, Lincoln’s proposed leniency with the South might have reconciled the country. But his assassination made that leniency politically un-achievable.