> Most northern US states abolished slavery even before Britain, France, Portugal, and (especially) Spain did
Sort of. France and England abolished slavery within their own territories before the US states did, but it did not extend to their colonies until later. France banned slavery within its home territory back in the 1300s (Free Soil Principle), but continued with slavery through its "Code Noir" (Black Code) in its colonies, where slavery was not permanently abolished until 1848 (it was abolished at the time of the French Revolution but then reinstated by Napoleon). England abolished slavery at home in the mid 1700s but not in its colonies until 1834.
(Like the US North, England and France had very small populations of Africans/others, so it was relatively painless and easy to ban slavery there, while continuing to accept slavery "elsewhere". For the US North "elsewhere" was the South, for England and France it was their colonies. Same principle though.)
Along the same lines, slavery of Catholics was forbidden in Europe all the way back in the Middle Ages. So it was acknowledged to be something bad, that Christians should not do to each other. But slavery of "infidels", heathen/pagan/Muslims/etc., was OK - and not only okay but sanctioned by Romanus Pontifex in the 1400s granting Portugal the authority to enslave "pagans" (basically all non-Europeans) along the coast of Africa. (Incidentally, as some Africans converted to Christianity, this posed a problem (they were no longer pagan and couldn't be enslaved), and so eventually it shifted to being about race and skin color rather than religion.)