The Union never cared about Africans, at least not the majority of them. It was just a whitewashing of their history that makes people think otherwise.

They fought to say "nu uh, you can't leave we own you" to the South, which the obvious irony should be palpable even now. Which is why said whitewashing took place, because on almost no axis except the "slavery" one does the North not come across as the aggressor and in the wrong.

This is all to say they'd still do all the did during the War regardless, even if they killed all the niggers they came across.