The Bad Seed

The central character of the story is Christine Penmark, a young mother who finds out that her eight-year-old daughter, Rhoda, is a murderer. Prompted by strange dreams and her landlady's dalliance in psychiatric theories, Christine begins to wonder if she's an adopted child. She eventually figures out that she is actually the sole surviving daughter of "The Incomparable Bessie Denker," a well-known (fictional) serial killer. Bessie Denker's career is based very roughly on the real-life careers of Belle Gunness and other such black widows. The description of her execution in the electric chair is based on that of Ruth Snyder.
Rhoda is portrayed as a sociopath, although the term was not widely used at the time. Like her grandmother, she has no conscience and will kill if necessary to get whatever she wants, whether that be a penmanship medal she felt she should have won, the silence of a janitor who knows more than she wants him to or the desire to possess an opal pendant. By the time Christine puts the truth together, Rhoda has already killed three people (the old lady who was going to leave her a snowglobe, Claude Daigle, and Leroy the janitor) and one puppy. An adept manipulator, she can easily charm adults while eliciting fear and repulsion from other children, who can sense something wrong with her.