It is far more difficult to root out corporate sin, and far more difficult to address the individual culpability for that corporate sin. Likewise, the understanding of the violation of human dignity inherent in slavery came to be better understood through the time of the Renaissance.
It was Bartolome de las Casas who articulated a position against slavery in the 16th Century (first only against enslaving the Indians, then later he developed his view to be against the enslavement of anyone, including Africans). Think of the social sins of our own time (including but not limited to abortion, wages that don't match a living wage, environmental destruction, treatment of migrants)—not everyone is equally capable of changing them, and not everyone agrees on the particular solution. For that matter, not everyone even agrees on what the problem is.
In the same way, culpability shifts with understanding. Slavery was always objectively evil... however, it was not always understood to be so, therefore someone in the 14th Century is less culpable for their participation in societal structures that foster and depend on slavery than someone in the 16th, someone in the 16th less than one in the mid-19th, someone in the mid-19th Century less than someone today.
My whole point in bringing up the Founding Fathers at all was to point out the absurdity of the black-and-white thinking on the issue of secession and how slavery ties into it—I did not expect an actual answer to what I understand to be a difficult question, particularly read through a modern lens. Both in the Revolution and the Civil War, slavery was an issue, but not the only issue.
The preservation of slavery in the US during the Revolution doesn't make the Founders evil. It makes them complex; not all wanted slavery, but at the time they were also unable to abolish it.
Remember that George III had no higher moral ground than Lincoln in his emancipation of slaves—it was about economically harming a rebellious political faction, and nothing more. Slave trading was legal in the British Empire until 1807, and slavery was not abolished in most of the Empire until 1834.(Wikipedia)
History will likewise judge the ignorance and/or inaction of our own era's handling of certain issues.