Why were southerners particularly violent people back then?

[Del Tachi]

Honor culture, derived from the culture of the Scottish Highlands where the honor of oneself and one's clan were of paramount importance, was the main reason. You can still see echoes of this today even though it isn't as overt.

If we're making the assumption that the South's Culture of Honor is a strictly colonial import (which is not 100% the case, I believe) then this explanation misses a really big part of the story in that it doesn't account for the traditions and culture of the Southern planter aristocracy, who were mostly from the South of England or France instead of the Scottish or Welsh Highlands.  The planter class was much more influenced by feudal ideas of chivalry and self-restraint than the clan dynamics of the Scots-Irish.

What you're describing is the "Hatfield vs McCoy" type drama that defined rural upland culture from Oklahoma to New York (i.e., not a Southern cultural artifact at all.)  The Culture of Honor that defined the Southern planter class is of an entirely different typology, even if our contemporary understanding of the region glosses over this important distinction.     

[Del Tachi]

For example of a different cause: Nathan & I have written previously on here on a sociological “trend” of racism shifting to the upper South, particularly Appalachia, and then increasing in the North & West of the country relative to the South. It seems as though at least part of the reason for this is the larger black population in the South relative to the rest of the country.

I think it's the inverse.  You're seeing "more racist" attitudes in the Northeast and Midwest because they, like the historical South, are becoming more non-White.  This is especially the case in "legacy" industrial cities and inner-ring suburbs that were once the epicenter of middle-income, White America.  Diversity must exist in a place prior to there being any observable racial conflict, after all, and that diversity is something Yankee parts of the country largely missed out on historically.