There's more than one way to explain zombie movies. Apply post-modern literary theory to zombies and you can end up with all kinds of crazy shit. Stephen King wrote that zombies represent the fear of cannibalism. The explanation at the current height of intellectual fashion equates zombies with homeless people.
It originated as an implication in the 2002 British zombie movie 28 Days Later, drawn with shots of shopping carts. Later, the South Park episode Night Of The Living Homeless made the comparison explicit. They replaced the zombies' dispirited moaning for "brains" with moaning for "change." The analogy also extended to the absent-minded, vacant, shuffling walk that zombies and (some of) the homeless share.
But there's something sick about demonizing the most unfortunate people in modern society, and I don't think that's really where 28 Days Later was going. I think 28 Days Later compared zombies to the homeless as the if part of an if-then argument about fascism in the United Kingdom. As an American with British parents, I've seen British communication fail this way many times. The way that the British say "if" is pretty easy to miss.
Another problem: saying that "zombies equal homeless people" gives us a simple, pat explanation. Simple, pat explanations are useless when you want to understand a mythological monster. Mythological monsters have more than one dimension. It's their multi-dimensional nature that makes them valuable for stories. Consider vampires. True Blood can tell stories about sexual identity with vampires, Twilight can tell a story about teenage romance with vampires, and Let The Right One In can tell a story about isolation, co-dependent relationships, and disease with vampires.
There's not much of the homeless in the zombies from George Romero's unsettling, brilliant Night Of The Living Dead, which every zombie movie since 1968 owes a debt to. The zombies in Dawn Of The Dead, Romero's 1978 sequel, are nothing like the homeless at all. They wear nice clothes and spend every moment of their un-lives wandering around a mall looking at brand-new stuff. They look just like shoppers.
28 Days Later and Night Of The Living Homeless tell stories about homelessness with zombies, while Dawn Of The Dead uses zombies to tell a story about the emptiness of consumerism as a cultural value. This story remains relevant today, especially now, when we're discovering the emptiness of consumerism as an engine of the economy. The low-budget horror-comedy Re-Animator, which blends Ghostbusters with Moby Dick, uses zombies to tell an incredible story (by H.P. Lovecraft) about obsession, and the dangerous border territory shared by madness and genius.
Simon Pegg - who starred in and wrote Shaun Of The Dead, a hilarious zombies spoof - says zombies represent the fear of death itself:
As monsters from the id, zombies win out over vampires and werewolves when it comes to the title of Most Potent Metaphorical Monster. Where their pointy-toothed cousins are all about sex and bestial savagery, the zombie trumps all by personifying our deepest fear: death. Zombies are our destiny writ large. Slow and steady in their approach, weak, clumsy, often absurd, the zombie relentlessly closes in, unstoppable, intractable.
However (and herein lies the sublime artfulness of the slow zombie), their ineptitude actually makes them avoidable, at least for a while. If you're careful, if you keep your wits about you, you can stave them off, even outstrip them - much as we strive to outstrip death. Drink less, cut out red meat, exercise, practice safe sex; these are our shotguns, our cricket bats, our farmhouses, our shopping malls. However, none of these things fully insulates us from the creeping dread that something so witless, so elemental may yet catch us unawares - the drunk driver, the cancer sleeping in the double helix, the legless ghoul dragging itself through the darkness towards our ankles.
That's the best explanation I've ever read for the terrible disturbing power of Night Of The Living Dead. The movie packs an extraordinary punch. Roger Ebert reviewed it when it was in theaters, and got very upset that kids had been allowed to see it:
The kids in the audience were stunned. There was almost complete silence. The movie had stopped being delightfully scary about halfway through, and had become unexpectedly terrifying. There was a little girl across the aisle from me, maybe nine years old, who was sitting very still in her seat and crying.
I think Pegg is right that Night Of The Living Dead uses zombies to tell an amazing story about human mortality. But I don't think that's what zombies are about. This might be glib, but it's also guaranteed to be true: zombie movies are about zombies. That's the only thing they're guaranteed to have in common, however, because zombie movies can use zombies to tell different stories about different subjects.
Sometimes the only thing you can say with accuracy is that many generalizations are false.
Consider what Pegg says about vampires and werewolves - vampires are all about sex and werewolves are all about bestial savagery. It's not true. There's nothing sexual at all about the vampires in 30 Days Of Night, and the game Werewolf uses the werewolf mythology to play with suspicion, paranoia, distrust, and logic. Bestial savagery has nothing to do with it.
As another example, and to vent, I recently accused somebody of being a "werewolf," in the sense of that game, and based my main argument around a critical comment which that person had made. Some people responded that they felt the critical comment was unintentional. This drove me crazy. I wished I had not even wasted my time communicating with anybody that simple-minded in the first place.
Everybody knows that the werewolf doesn't find out that they're a werewolf until halfway through the movie. Unintentional violence appears in nearly every werewolf story. If my argument is that X act was a werewolf-y act, and the counter-argument is that X act was unintentional, that makes no difference at all. It is no counter-argument. It only serves to illustrate that some people didn't get the metaphor. You might as well say, "I like turtles."
The most common werewolf story is a story about bestial savagery, maybe, but the werewolf mythology has additional dimensions. It makes a great context in which to tell a story about unpleasant people who do unpleasant things and think of themselves as being much nicer than they really are. Imagine a man with children to feed and a sick wife who will die without his help. Everything hangs on him. And then he finds out he's a werewolf. He's been killing people every full moon for six or seven years when he finds out. It's the 1600s - he's a settler in an American colony, far enough from other people to be isolated from the deaths of strangers, especially at the distances a wolf can run in a night, but close enough to find fresh meat every time the craving hits him.
This is a religious man. He believes in God. He prays every day. The insanity and hostile politics of evangelical Christians have made religion seem a shameful thing in America, but this is no bigot - he seeks with all his heart to serve God and to do good in the world.
What does he do now?
Does he give himself up to his church, or the witch-hunting law enforcement of his day, and die at their hands with a clean conscience? Does he keep his freedom, even though he knows it means he will go on killing strangers? In the 1600s, in America's colonies, life was hard, and women and children were often dependent on men in a way that is hard to even imagine today. If he dies, his wife and children die. If he doesn't, he'll be killing innocents for years to come. Whichever decision he makes, somebody innocent dies. It's a story about a werewolf that has nothing to do with bestial savagery, and everything to do with responsibility, regret, guilt, denial, and the fact that looking yourself in the mirror can require that you make the right decisions in difficult circumstances.
Like Bobby Fischer said, sometimes there are no good moves, but there is always a best move.
I recently wrote a screenplay for a zombie movie. It's not about human mortality and it's not about homelessness. In fact, I had the male and female leads both hug a homeless guy to hammer that point home. My zombie script began as an idea for a cyberpunk presentation of The Crucible by Arthur Miller. It became a response to Clay Shirky's Here Comes Everybody, a brilliant book that downplays the dark side of its argument. My zombie script is about YouTube. It's about ostracism, grief, how technology changes society, the tension between chaos and order, and, more than anything else, it's about zombies killing people, and bad-ass survivors shooting a lot of fucking zombies with a lot of fucking guns.
Update: Alan Francis tells me shopping carts aren't associated with the homeless at all in the UK - UK homeless don't have shopping carts, just plastic bags. So the 28 Days Later homelessness thing appears to be an American interpretation of imagery which in the UK means something different.