Assuming as Lee's cousin Samuel Philips Lee who fought with the US Navy did. Or as Lee's brother, John Fitzgerald Lee who served in the US Army Judge Advocate General Corps did. Or as Lee's sister Anne Lee Marshall and her son who served in the Union Army did.
It really was a war of brother against brother. There was no established belief that loyalty to a state overrode loyalty to the country -- especially not among career soldiers and sailors. I suspect in Lee's case, it wasn't so much state or family tradition that carried the day. If he fought for the United States, he'd probably be fighting against his own children, and he couldn't do that.
It was a tragic choice he made. But it wasn't the right one.
Is Lee, who fought well and bravely for the Confederates more of a traitor than McLellan, who ran on a platform of reconciliation with the South, against Lincoln, and who’s policy of war directly lead to it’s extension?
That would be an easy yes.
Politicians will always argue about whether to fight wars and how to fight them, but to take up arms against one's own country is something else altogether.
So why is the opinion of Lee’s cousin as to the ‘correct side’ of greater significance to the opinion of Lee himself?