I posted this a few years ago on the subject:
Well, “death” would be the obvious answer. The zombie horde is an in-your-face reminder of your own mortality, without any sort of reassurance of an afterlife or eternal reward. Ghosts, for example, are scary, but they’re also evidence for the existence of a soul, that after we die, some part of us goes on. Vampires, at least in the Stoker mold, are the same. If Dracula is damned, then that means there must be grace for at least some of us. And his reaction to a crucifix indicates that there’s some validity to the concept of a Christian afterlife. The more modern “vampire as protagonist” stories that jettison the religious component of the vampire myth are merely replacing it with a different sort of immortality. Sure, a vampire can kill you, but it can also bring you back with superhuman abilities, eternal youth, a trendy black wardrobe, and all sorts of other great side-benefits.
The zombie horde is different. There’s usually no supernatural background to it. The zombies are either the result of science gone awry, or are presented with no explanation at all. It brings us face-to-face with death without any comforting evidence that the universe is anything other than random and pitiless. It also presents death in a way few of us like to confront it. There’s always something faintly ridiculous about zombies. They aren’t sexy and cool, like a vampire, or mysterious and otherworldly like a ghost. The archetype of the zombie horde isn’t a charming European aristocrat or a beautiful spectral woman. It’s an overweight housewife with a pink teddy bear sweater and a gaping head wound. Like real death, being turned into a zombie robs you of your dignity, and turns you into something that is at once ridiculous and repulsive, like the pile of zombies trying to claw its way up the down escalator in Day of the Dead. This fusion is so effective that even movies that are supposed to be comedies, such as Shaun of the Dead, are still highly effective as zombie movies.
The other potent cross-fertilization in this genre is the plague aspect: death as contagion. It’s not enough that the zombies kill you, they turn you into one of them. And not through ritualistic seduction, like a vampire, but by random idiot chance. A superficial bite wound, often received during a moment of victory, and the sickness is in you. You’re already “one of them,” even though you’re not dead yet. And then there’s nothing you can do but wait until you turn on the rest of the survivors, or put a gun to your head and end it quickly.
Lastly, there’s the human commentary. The zombies aren’t evil. They’re a force of (un)nature, apocalyptically deadly, but wholly unmotivated by desire or intent. Essentially, a human-sized virus. But the destruction they entail creates a backdrop in which “normal” humans become monsters themselves. The naked confrontation with death inevitably brings out the basest reactions of the survivors. Faced with the total collapse of society, people quickly revert to the worst sorts of barbarism. Essentially, the zombie horde destroys you even if it never catches you, by revealing you to be an uncivilized animal who will violate any principle or ethic in order to survive. Although, like all monster movies, the zombie apocalypse film is ultimatly moralistic: the characters who most eagerly pursue their own survival over the common good of his fellow survivors, the more graphic his eventual demise at the hands of the zombies will be.