"You do know that he was a decorated member of the colonial army before becoming a turncoat, right?" Um, yes.

The analogy ends there. The US government made a conscious decision to give amnesty & pardon those who fought for the south. Lee effectively ended the war by surrendering and kept it from restarting 2 years later by talking down those southerners who wanted to. He supported Johnson's plan for Reconstruction & became a symbol that aided its success. He was a productive, loyal member of American society the rest of his life.  He was neither pro-secession nor vehemently pro-slavery. He was a complex man (supported black education but not the right to vote); a product of his time. 

Where Arnold was ticked off at America & out for revenge, Lee fought the Union out of loyalty to his state, which democratically voted to secede. When he had the chance to bring the war to a close, he took it. Yes, he fought against his former country. But simplistic analysis of this man is impossible.