I think it interesting that the South, particularly its soldiers, eventually (if begrudgingly) accepted the unified nation. While it was not so much let “bye gones be bye gones” the collateral and longer term strife it produced is today pretty much an artifact. Apparently such is not the case for the legacy of slavery which was largely a national institution at its inception.
When the Colonies exercised their natural right to secede from England, every colony was a slave state. They were *ALL* slave states.
It was actually the words in the Declaration that started the ball rolling towards abolition. The State of Massachusetts explicitly cited the Declaration of Independence in the "Freedom cases" which overturned slavery there.
Pennsylvania explicitly rejected that argument, though.