The Bad Seed (1956)

To paraphrase Monty Python: she's not the Antichrist, she's just a very naughty girl. If the tightly-wound pigtails and goody-two-shoes demeanor of junior miscreant Rhoda Penmark weren't disturbing enough, there's also the small fact that she's a sociopath -- devoid of a conscience and responsible for several cold-blooded murders no one would ever suspect her of.