Camilla

Camilla is the main antagonist of the 1990 horror film, The Guardian - she poses as a babysitter in order to get close to an infant child whose parents require her aid: however as the movie progresses the parents find themselves in a desperate struggle to save their baby from Camilla, who is revealed to be a demonic dryad that seeks to kill the baby and add its soul to a collection of sacrificed infants she has taken over the course of centuries and added to the trunk of a horrific tree-demon (which is also a part of herself).

The Guardian is one of a relatively few horror films that focus on an antagonist that actively seeks to kill an infant and although Camilla ultimately fails in her task she is still shown to have been a serial baby-killer, arguably making her one of cinema's more dark and geniunely twisted monsters.

Camilla's ultimate fate was to die when the baby's father finally cut down the monstrous tree-demon that made up Camilla's "true form" - causing her to also die in a gruesome fashion as she was linked to the tree (a reference to the myths of dryads in ancient Greece).