monsters as villians

I created a monster that I'm considering as the center-point of a story. You'll need a bit of detail to get how this works. In this 'verse, vampires are hybrids with both human and demonic physical aspects combined into one body, but the human aspect can die off, leaving a demon beast that pretty much cares for little more than killing. Using Beowulf as a metaphor, vampires would be represented by the settlement that Grendel decimates on a regular basis while the demon beast would then be a lot like Grendel. My monster has been genetically engineered off of vampire stock: she can pass, based both on appearance and how she interacts, as human but her core motivation is that of a demon beast: kill, kill, kill.

I was trying to work out why I couldn't come up with a story around this monster and I realized that many – although not all – of the classic monsters have a human component. In The Mummy, Boris Karloff's character wants to restore his ancient love to life. Lon Chaney, Jr.'s character, in The Wolfman, anguishes over the destruction caused by his other self, as does Dr. Jekyll over Dr. Hyde's actions. Darth Vader reveals himself as Luke's father and defies the Emperor to save his son. The Emperor, as a minor character, gets away with being purely evil.

Dracula, on the other hand, is a more interesting case. He presents himself as human – in the old Bela Lugosi film he's quite suave and sophisticated – but he doesn't care about anyone other than himself. However his minion, Renfield, bridges that gap: he acts as both monster (devouring both flies and nice juicy spiders) and man (advising the hero to get “the girl” as far away as possible before the sun rises).

And then there are, as I suggested earlier, monsters that seem to have no humanity. Zombies tend to fall into this category. Novels such as Warm Bodies, where zombies are becoming human again, aside, most zombies aren't much more than human-looking eating machines. Ming the Merciless cares for no one other than himself. His daughter takes on a role similar to that of Darth Vader – switching sides near the end of the movie – but she doesn't fill that gap as effectively as Vader or Renfield. Godzilla tends to just rampage through Tokyo, although I do find it interesting, in this context, that my favorite of those monsters is Gamerah (the big flying turtle) who is, if I'm recalling it correctly, a friend or helper to humans.

These are mostly ramblings I had last night. What am I missing about what makes a monster into a great villain?