The conceptualization was "brother vs brother" not "good vs evil" for most of American history.
If nothing else, just remember that Mario there would shift from defeating the slaver states to genociding his way through the Great Plains, it was arguably the centralization and militarization required to win the Civil War that enabled the conquests of the late 1800s.
all of those are wrong, too. The Mason-Dixon line separated the northern and southern states of the original 13 colonies. It has nothing to do with slavery.