Is Slavery Just This Greater Evil?

The idea that slavery would eventually fade away wasn't, well, i mean, it it was wilfully naive, but is perhaps not quite as wilfully naive as it seems looking back. I guess the counter argument to our modern morality of saying, while slavery is just this greater evil, would be, then what would happen? You'd have sort of two united states emerging, and the north would sort of be in a race out west with the other one. What on earth would foreign policy look like with that regime? Would you try and prevent their import of slaves? I've no idea, but you can imagine it going wrong and you ending up with something a bit like the civil war