Why do you suppose Lee thought slavery was a greater evil for whites than for blacks? After all they weren’t the ones who were property being bought and sold.
Thomas Jefferson had thoughts along the same lines. He believed owning slaves necessarily corrupted the slave owner. I would look up some quotes, but I have to get to work...
He took the same position as some of the Founding Fathers. There were, however, several notable black slave owners at the time of the war.. His name escapes me at the moment but one notable one was in Louisiana.
We tend to look at The War Between The States in a 21st century context when we should look at it in the context of the 19th. Pre war Americans primarily considered themselves citizens of their respective states.
With that in mind why do you suppose Lee fought for the Confederacy? It certainly wasn't to preserve slavery.