U is for Undead

There are no greater villains than the undead.
In one way or another, they embody everything we fear about ourselves, and what may become of us.

Ghosts
Many of us already feel that we only partially exist in our own lives.  Mindless, repetitive, obsessed, that's what it is to be relegated to haunting.  To be a ghost is to have lived a life that was so miserable, it is impossible to rest; even in death.

Ghouls
We've all had desires that have harmed us;  needs that we wanted met so badly that we hurt someone in the process.  Most of us feel bad about it.  We fear being the kind of thing that shamelessly pursues its own wants with complete disregard to all else.  That is what it is to be a ghoul.  You have a hunger, or greed so strong that you will enslave yourself, and kill anyone or anything to satisfy it.  But it never stops.  The hunger never goes away.

Revenants
Your life is over.  You're not blessed with the ignorance of the mindless undead.  You remember what happened.  You remember in full, vivid detail the terrible, violent events that led to your untimely death.  You relive it constantly.  You cannot rest until you have revenge.  Maybe then you can rest.

Vampires
More than any of the others, vampires are the most human.  They can look and act the part, but they are monsters.  They are cold-blooded predators in the bodies of someone else' victim.  We love to humanize them, to make them feel.  That makes them less terrifying, even sympathetic, but it's a lie.  Vampires are what we fear about sociopaths.  Someone that looks and acts just like us, but neither understands, nor cares about us.  They have their own agenda, and it is directly in contrast to our well-being.  There is no reasoning, or talking it into a better, more peaceful life.  You are its food.

Zombies
This is the ultimate representation of the fear of decay.  Death comes for us all.  It makes our loved one's rot away.  That unnerves us because we know that one day it'll be us putrefying, so we bury it.  We get to forget for a little while longer the terrible fate in store for us.  Only zombies won't let us forget.  They creep after us, making us see what the future holds.  They want to make us like them, and they will never, ever stop.

But nothing will ever beat dancing skeletons.