To be fair to the fictional sheriff and his posse, that's all subtext. I don't think there are any lines that give a direct suggestion that the men are racist, or that they even saw that the movie's lead character was black before they shot him.

It's notable that the role of Ben wasn't specifically written for a black actor. It was a genius (and daring) bit of casting, but presumably white, uneducated truck driver Ben from the original script meets the same fate. They definitely played up the imagery afterwords to reinforce the idea that some members of society can't look to the rest for rescue, certainly.

I think the Vietnam subtext is quite strong there as well, with the troops moving through the countryside fighting hostiles with little regard to who is civilian and who is an enemy.

Future zombie films would play up these angles with lynchings and taunting of the dead, and callous, armed members of the living enjoying the violence they get to dish out, losing their humanity in the war against the unliving. Night of the Living Dead played things pretty straightforwardly though.