IIRC he viewed the North as representative of the "bourgeoisie" and the South as a feudal system that needed to be swept away before the glorious Communist revolution could occur. A historical progression from feudal to bourgeois to the Communist end game sort of thing. Kind of like the way some Christians think you need a Tribulation before Jesus can return.
Another fun thing to point out is that the mustache man was also a Union supporter, at least if the Hitler Table Talks can be believed. (And there's quite a few skeptics, TBF.) In any event, the idea of "States' Rights" is an absolute anathema to a National Socialist; one of the first things the Nazis did upon assuming power was strip the last bits of local authority that had been retained by the formerly independent states after the German unification. We can say that much. Shirer goes about it in detail; they did things not even Bismarck would have considered.