I had ancestors from Massachusetts and southern Kentucky soldering on both sides during the Rebellion.
Lee waged war against the United States - the Constitutional definition of "treason." Fortunately, for the Rebel leadership and for the nation, Stonewall types were not in charge on the U.S. side when it all ended in 1865. Even Jefferson Davis was spared, perhaps in gratitude for his management of the war.
As for your "just" "cause," the citizens of Maryland and Pennsylvania whose property was looted in 1863 and 1864 in return, at best, for worthless rebel script would disagree