Trying to portray the founders as remorseless slavers ignores historical context. It's taking something that was everywhere and trying to pretend that it mattered so much more in a particular place. Further, that the particular place was at the forefront of abandoning slavery only makes such remarks all the more out of place. Lastly, trying to claim that the founders, today, would be disappointed that slavery is gone is nonsense. One cannot simply pull someone out of basically ancient history and pretend they'd be the same today.