Robert E. Lee was anti slavery and sabotaged the Confederacy.
Lincoln offered Lee a high level command position, but Lee turned it down knowing he would still be subordinate to generals he thought to be incompetent or ineffective such as Scott and McClellan while the Confederacy had sharp tactical minds such as Jackson. Lee decided his best course of action was to join the Confederacy as a top ranking general and sabotage things from the inside.
It was easy to do. He maintained a close relationship with Jefferson Davis and when the chance to lead the Army of Northern Virginia opened up, Davis appointed Lee. Lee, wanting to dramatically shorten the war, quickly put his plan into action. He would lead an invasion into the Union and split his army, but quietly leaked his plan out to the Union army. He was hoping his army would be surrounded and forced to surrender, putting a quick end to the Confederacy. Instead, the incompetent McClellan hemmed and hawed over what to do and simply fought a damn bloody battle. Lee's army had to retreat. He would go on to lead an incursion into the North again a year later, but the Union still could not decisively end the war quickly at this point.