The original US Constitution is explicitly pro-slavery. Not only does it explicitly require non-slaveholding states to return fugitive slaves to their oppressors, but it has multiple mechanisms intended to ensure the dominance of slave states in the federal government.

The Constitution was never a unified idealist vision of liberty. It was a grungy political compromise between factions that did not agree on what the country should be. These included New England Puritans (religious cultists; but abolitionist), New York Dutch bankers (who wanted the money back they'd loaned to the states), Southern planters (patriarchal rapist tyrants), and Mid-Atlantic Quakers (pacifists willing to hold their noses and make peace with the Puritans and planters).