In 1951, celebrated Golden-Age Sci-Fi author Cyril Kornbluth* published in Galaxy magazine the story "The Marching Morons". In the fifties, Sci-Fi fell into two fairly narrow categories: pulp sci-fi was all about scare and schlock, alien invasions and monstrous creations. Pulp sci-fi was for the drive-in, and both catered-for and nurtured the public's fear of the strange, unknown, foreign and, of course, commie. Highbrow Sci-Fi (which later became literary SF) originated in the social sciences, indeed I've heard Sociology claimed as the only true science. Kornbluth was resolutely literary and sociological, and if you haven't read Morons, go do so now.
Let's take a look at the first two really big Zombie Movies. These were the main influences on modern culture, with regard to the Zombie. They are the equivalent of Bram Stoker a source material for Vampires.
George A. Romero's seminal work has a small group of outsiders fighting off the numberless dead, ultimately trapped in an isolated farmhouse they are devoured one by one. The symbolism is much the same as Kafka (in The Trial or The Castle) or Camus (most notably in The Outsider). The dwindling group of survivors are you and me. The living dead are Kornbluth's Morons.
This is brought home ten times harder in Dawn, which is set principally in a shopping mall, and most of the Zs are dead shoppers.
As human animals, we sometimes have a hard time seeing strangers as actual people, and when those strangers are innumerable, they become entirely faceless, voiceless, doing nothing other than crowding us out, consuming everything, overwhelming our planet's (and our) meagre resources with no knowledge or understanding of the consequences of their existence, and worse, the consequences of their proliferation.
Zombie stories, with the exception of a few challenging and esoteric tales, fall into two broad categories. Those where there are some survivors at the end, and those where there are no survivors at the end.
In those where there are survivors, there is a certain wish fulfilment. The wish is to have the world to ourselves and our personal, carefully selected group of survivors (like Drax in Moonraker), and also to have free license to slaughter the Morons.
Those where there are no survivors are for those who can't stomach the necessary sociopathy of survival, and hence realize that the overwhelming human population will ultimately destroy the earth, itself and them.
Zombies are in the zeitgeist right now because now as never before, more and more people are aware that even if overpopulation isn't our number one problem, then it is the largest, and most severe exacerbating factor. Never before have we been closer to Kornbluth's moron apocalypse.
And it will take a psychopath to save us. Good thing they're 1% of us.
*this name seems to have lots of pronunciations (cornblue, cornblute, cornbluff, cornblooth, cornbluth) so take your choice