Thousands and thousands of years later, you'll see that exact same image on the covers of zombie novels and movie posters: the rotting hand, clawing up from the ground. It's stamped into our consciousness.
For the Love of God, Why?
People back then already suspected the dead were a bunch of douchebags because anyone who hung around rotting, oozing corpses tended to get sick. Not knowing what microbes were, they declared the dead (and areas where the dead hung out) to be "cursed" or "haunted." So already people knew corpses were something to be afraid of.
But people who study things like zombies say there is a psychological element; that seeing humans turned inhuman raises some really awful doubts about human nature. What if all of the things we think make us human--our thoughts, our feelings for our loved ones--is all pointless bullshit? After all, even insects live, eat, have sex and build things, and they do it all without "minds" or "souls" or even a simple high-five to celebrate a job well done. So all that stuff we think makes us special... what does it matter?
So, when confronted by a human who's been transformed into a mindless, shambling predator, the terror is in the sneaking suspicion that we're looking into a mirror.
Jesus. Let's pause for a boob break.
But then you have to take into account...