Treason? It’s my understanding that succession was an acceptable alternative until the Civil War removed that option.
By the way, my ancestors arrived and settled in Richmond, Virginia just a few years before the Civil War. Because of where they lived, one had her house burnt down and others had their supplies and livestock stolen from them by the Union Army. The Confederate Flag served as a reminder of resistance, not necessarily to the Union, but more so to the deplorable conditions that existed in the South after the war. Southerners look at the Confederate Flag and remember they resisted, fought a war and lost, but survived and are still here.
It was treason. It was the taking up of arms against a duly elected government. When Lee invaded the North his army stole and looted too. Whats your point? And if the South had won the war it started would it have ended slavery?