Southern states by and large seceded over fear that the northern states, which outnumbered the south, would outlaw slavery throughout the country. That's clear from most of the declarations that southern states passed to formally state their reasons for leaving the Union. So it's not true, as sometimes claimed, that the primary reason for the war was "states's rights" (well, yes, but the right of a US state to maintain slavery), or "northern capitalists keeping the agrarian south under their heel".

But that doesn't explain why the average southern soldier—who didn't own any slaves (that was for the rich planter elite minority), who probably personally knew and possibly were friends with slaves, fought and died for the Confederacy. Claiming that "the entire southern United States were overcome by an irresistible race hatred" is as ridiculous (not saying that you are claiming that; you recognize that such a statement is silly) as stating that "every German soldier in WW2 was a Nazi", which was incorrect on both a literal sense (in terms of being a Nazi Party member) and a figurative sense.

Ultimately the average southern soldier fought and died for the same reason the average German soldier did: For his home (and homeland), his family, his friends and comrades, his way of life. Nothing more or less grand than that.