In my opinion, the major sin was not slavery, which was a normal estate of man from earliest history. True, it was incongruent with the strongly asserted fundamental reason for forming the country. The Founders clearly recognized that fact and came to a gentleman's agreement (broken by southerners) to do-away with slavery after the demise of the southern founders. I think Washington was the only southern slaveowner who followed that agreement, but I could be wrong.

To me, the greater sin was post Civil War. The war settled the issue, the Constitution codified it, but the nation generally rejected it. Some de jure (mostly southern states) and some de facto (nearly everywhere else). The period when equal rights were the standard but not enforced by law, nearly 90 years, is the greater shame on this country. Politically and philosophically we knew better, as a people but did wrong. It took the courage of civil rights workers, especially the NAACP-affiliated groups and various Christian denominations (notably, Quakers) to move the people to where they should have been in 1865.