Dixon was there mostly as a character foil for the black guy. Plus, he was there to Frank can say the whole "there's no blacks, white or rednecks" thing. And finally, he was there to demonstrate that the human survivors can leave living humans behind as be the very monsters they knew the redneck was.
I realize this, but it's almost like the writers took the easy route by making the guy an Evil Racist. It would be a lot more dramatic and heart-wrenching to have to leave behind a good guy.