"What scares you is really all you can write about. Most horror writers are terrified of everything, so it gives them a big scope. They're so focused on a milliion ways they could die, real and unreal."
Stephen King says his greatest fear is getting Alzheimer's, and I think you could make the case that zombies are a stand-in for that fear--being yourself yet no longer yourself.
What's your greatest fear? Perhaps writing a horror story might be good therapy.
(What writing advice did the greats have for us--masters like Twain, Austen, Dickens and the top modern authors as well? You'll find it in my book, "Your Creative Writing Masterclass." It's published by Nicholas Brealey and available from Amazon.com or your other favorite book seller.)