On a certain level we all knew it already because we all knew he was a life-long slave owner. I find it somewhat interesting that reading Jefferson talk about it in his own words seems to be making it real for people (didn't we already know how terrible he was on this count when we first learned that he owned human beings?).
As for his legacy, I think the point some people are circling is that while there's quite a bit we don't like about Jefferson there is a lot to like about Jeffersonianism - that special brand of American liberalism that belongs to Jefferson as genuinely as his slave ownership does. There's certainly a lot I like about Jeffersonianism.
In that sense, I think we file Jefferson with Heidegger. Which means people are probably never going to ultimately agree about him.