This yields two basic kinds of horror from the zombie story–one makes it kin to Rhinoceros: it’s about the rise of fascism/nazism/&c. Formerly intelligent, independent, real human beings lose their self by becoming infected with [zombieness], and become part of a mindless horde that regularly engages in activities that individuals would before have found horrific. It is a metaphor for the mass, irreedemable loss of the minds of those one loves. (And this is the nature of horror: losses must be irredeemable. If you are in for a penny, you need to be in for a pound.)I've never much liked the wave of zombie infection = rise of wicked ideology theme. I'm not disputing he's right about zombies being used that way, but it's always seemed to miss the point to me. Part of what makes it so scary when previously normal people do horrific things in the name of a cause is that they don't become irredeemable inhuman. When Zimbardo and Milgram's subjects finish up they go back to being normal undergrads; when Colonel Saito wasn't running POW camps in Indochina he was probably playing with his kids and taking walks with his wife and worrying about going bald, just like the rest of us.
The second is less about the psychological contagion and more about individual alienation: that the world is full of things that look human but behave atrociously drives home the idea that the individual is lost in a completely hostile world. Here the point is to set the hero, or heroes, at odds with a universe that is both familiar-looking and also completely alien. (Interestingly: the death of the main character in Night of the Living Dead, despite not being a zombie-related death, actually supports this theme; why? Because the movie is not about ZOMBIES, it’s about what zombies mean.)
It's exactly the ability of normal people to do monstrous things without becoming monsters that is so scary to me. It's easy to justify a world in which Bad Things are done by Bad People, it's much more difficult to accept that Bad Things are done by the guy who sings in the choir and calls his mother every Sunday and brings donuts into the office to share all the time. ((1) Yes, that's a reference to Dexter, which relies on this phenomenon and the fact that we're still undecided about vigilantism, and (2) No, I'm not saying there are no truly Bad People, just that I don't think they're frightening in the same way or as good of an referent for zombies as "normal" folk who behave badly.)
In zombie fiction you're always hearing things like "No, he's not your brother anymore. Your brother is dead, that's just his body, let it go." I don't think that's a good analog to someone who's joined up with the blackshirts or what have you, because one day that guy won't be a blackshirt anymore, and he'll probably go back to being normal in most respects. But as we know, you can't go back from being a zombie. The fact that people can compartmentalize so well that they can go and do evil things and then come home when the war or revolution or whatnot is over and go back about their lives is what makes evil in the service of a cause (as opposed to evil in the service of psychopathy or ego) so frightening.