The United States was not his country when he took up arms against it. He resigned his commission in the U.S. Army when Virginia seceded and joined the Confederate States of America. He fought for the CSA and defended his home state, Virginia, from an invading army.
I respect the fact that you disagree on these points. I believe today, as Lee and others believed then, that secession is a legitimate response when the Federal Government abandons the Constitution. The states have no will to attempt that kind of action today, even though the Federal Government today has gone so much farther in its overreach than it had in 1861.
It was not an easy choice for Lee to make. He was offered command of the Union forces. He resigned from his command two days after the offer was made, saying that he could not go against his home state of Virginia. He did not become head of the southern forces until General Albert Johnston’s death.
If Lincoln had accepted Winfield’s offer, the “war of succession” would have likely come to a quick end.