Nurse Ratched
If you've ever read the book One Flew Over the Cuckoo's Next, then saw the film adaptation, then you already know that Nurse Ratched is evil. She might pretend she's doing her job, but you can tell by the look in her eye, that she actually gets off on watching her patients suffer and pretty much lose their minds. By the end of the movie, she's responsible for not just one physical death, but the mental death of one of the best characters ever written. But I'm not giving away spoilers.