Lee had a choice. He chose to abandon his oath and his loyalty for the same reason that Arnold abandoned his: personal greed. Fighting against slavery and for abolition directly threatened his own wealth, as well as that of wife’s family. Had he fought for the Union, it would have meant freeing those slaves that kept him in a comfortable lifestyle and he wasn’t prepared to do that. He would not be the first general who threw in with a losing cause in order to enrich himself.