I feel Django Unchained and Hateful 8 cover the possibility of an interesting alt Civil War with it apparently starting a year earlier (probably due to Django's actions in the first film).
Another, slightly less obvious deviation is that repeater rifles seem
much more common than IRL. Where the type of mooks Django mows down during the Candieland massacre would probably have carried single-shot rifles, percussion-cap shotguns and the like, here they are armed Henry Repeaters in pretty large quantities. Of course, you could just chalk it up to just Candie being a rich guy that can spare the money to arm his goons with fancy new rifles, but even then they are still way too numerous for that.
That would mean the Civil War may have had more in common with World War 1 than what we saw OTL, as commanders would realize the hard way that line formations are suicidal with that kind of firepower. So, rather than march in formation at eachother, people would dig fieldworks.
Another likely consequence would stem from the massacre itself, as such a bloodbath would not go unnoticed by the Slave States and pro-Slavery advocates, meaning that the split would have been even
more severe, and even more motivated, than it already was, as they would believe they have to keep the Slaves in check, as setting even
one man free led to a mass-murder unseen in America until then, while Abolitionists would point to the massacre being a consequence
of Slavery as an institution, and that continuing it would only invite the Slaves to lash out even more. It would be an utter mess.
