*snip*
*sigh* But I am here so I might as well answer your question. Zombies are a Rorschach, they are what you want them to be. They can be terrifying if used correctly, they can be hilarious, they can be both, and they can be any enemy you want to kill without guilt. They can be political commentary, they can be social allegory, they can be you, they can be me.
In short, zombies are a blank slate of unnatural. They give the religious doubt and remind atheists of exactly what it means to believe in nothing. While vampires can make you feel special, zombies just remind you of what everybody needs to remember. You're just meat and you're not special thus this could be you.
Hopefully that was helpful in some small way.