I personally don't have an answer for you either. I live in SE Florida, which is like a second NYC down here, but as you travel North and West, there is that hatred. Even though we did serve in the Confederacy and have a lot of roots down here. I personally think it's there in that point of US history. I think they believe, that WE in particular ended slavery which is my theory, only because do you honestly think Lincoln would've ended slavery on his own?
I don't think we should accept ALL of the burden, considering they hate Italians down south as well. Now why, I have no clue there. My guess would be too ethnic, but even them with intermarriages. The blood isn't as pure as say 50-75 years ago with people getting right off the boats from Europe.