SFX magazine asked Adam Green, director of "Hatchet," the secret of making a good slasher movie. I think what he said applies to creating any kind of villain:
"You need to be able to sum up where they came from in a few lines and it needs to be quick and easy to explain and also tragic. It's no good just saying, 'Ah, he was born evil, so he grew up and killed people.' That's boring. There needs to be a sympathetic reason for why you have a killer: Jason drowned as a little boy and Hatchet's Victor Crowley was killed by his father. So there's a sadness there. That's what gets people invested in a good slasher movie."
Of course as you move away from melodrama into drama, these reasons become more subtle and they don't necessarily drive someone to murder, but perhaps to emotional violence. And the farther you go along that spectrum, the less difference there is between your protagonist and your antagonist in terms of what drives them. I always find books and movies in which that difference is slight to be the most interesting--and realistic.
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