Kelly's point on Lee being an honorable man raises the legitimate issue of whether we should judge people in the context of their time, or in today's standards.
Slavery was legal everywhere in the world through the vast majority of history. Democratic systems didn't exist for most of history either. Which means that most historical figures of consequence, on every continent (including Africa, and both pre-Columbian South and North America) were complicit in the maintenance of slavery and/or undemocratic rule.
So one way or the other, that includes Socrates, Themistocles, Marcus Aurelius, Alfred the Great, Saladin, Abd-ar-Rahman III, George Washington, and pretty much everyone else.
There's a difference between passively existing in a society where slavery is the norm, and devoting your life to the cause of preserving and expanding slavery. That's Lee's legacy, he chose it himself, and he should be held accountable for it when we look back on him as a historical figure.