Actually, he turned his back of his oath "to preserve, protect & defend the Constitution of the United States from all enemies foreign and domestic."

He was only enforcing the right of the states to secede, as protected by the tenth amendment (leftists love to pretend this one doesn't exist), which states:

The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.

Even Lincoln believed states had the right to secede, which is why he didn't actually invade until the Confederates attacked first.

He had no oath before God to defend slavery or Virginia.

I'd assume the Confederates probably made him take an oath before becoming a General, but perhaps not. Regardless, simple words cannot defeat the familial bonds one has with their home state — and it cannot alleviate one of the duty to serve said home state.

Article 3, Section 3 defines treason; "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort." He did this.

Actually, when the Civil War began, Lee was a citizen of the CSA, not the USA, thus was not committing treason, but was rather an enemy combatant.

He fought for slavery.

You say this again and again like it actually pertains to our disagreement, which isn't over slavery, but rather upon the merits of serving one's home state. You're like one of those idiots who scream "Black Lives Matter" when someone says they're pro-life.