Are you saying that slavery is The British Empire’s original sin?
It most certainly is NOT the United States’s - we can rule this one out completely.
Well, it's not our original sin in the sense that we invented it, because we obviously didn't. On the other hand, it is a sin that was with us from the original moment of our founding.
I agree that slavery was not the original sin and always had an opposition. When the North west territories were being settled slavery was outlawed there. The first 90 Years of America were largely spent in conflict over limiting slavery vs slaveries expansion.