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Rhoda Penmark is a fictional character in William March's 1954 novel The Bad Seed and the stage play adapted from it by Maxwell Anderson. She was portrayed by Patty McCormack in the 1956 film adaptation and by Carrie Wells in the 1985 made for TV remake.
Rhoda Penmark is an eight-year-old girl who, in the course of the novel, murders a classmate and a groundskeeper who suspects her. It is also revealed that she murdered an elderly neighbor the year before; by modern standards, Rhoda's commission of three murders qualifies her as a serial killer, although the term was not in use at the time of the novel's publication.
Despite coming from a loving home, Rhoda is a sociopath who is willing to kill to get what she wants. She is also a precociously talented con artist, adept at using a sweet, innocent façade to mask her true self from adults so they will give in to her. Her tricks do not work on other children, who sense who she truly is and avoid her.
March writes that Rhoda's evil is genetic; her maternal grandmother was also a serial killer, who also began killing at a young age (Rhoda's mother, Christine, was adopted at a very young age and so doesn't remember her real parents).
Character history (novel) 
Rhoda drowns her classmate, Claude Daigle, who won a penmanship award she feels she deserved, after beating him with her tap shoes. While no one initially suspects Rhoda, Christine notices that she seems startlingly untroubled by the other child's death. Christine, who has always vaguely sensed something wrong with her daughter, is troubled, but dismisses any possibility that Rhoda was actually involved in the boy's death.
The only adults who see through Rhoda are Leroy, the somewhat addled janitor, and, to a lesser extent, her teacher Miss Fern, who observes that she is a poor loser and rather selfish. Leroy spies on Rhoda and repeatedly threatens to "tell on her." Rhoda says no one would believe him, but begins to make plans to get rid of him.
Christine tries to relieve her fears by talking abstractly about the murder with her adopted father and Mrs. Breedlove, a neighbor who dabbles in psychiatric theories about personality. During the conversation, she recovers a long-repressed memory of her real mother, "the incomparable Bessie Denker", a serial poisoner who died in the electric chair. That night, Claude's mother arrives, drunk, at Christine's home stating that there is "something funny about this whole thing" and asks Christine to ask Rhoda about her last few moments with the boy. While Christine is locating Rhoda's necklace, which Mrs. Breedlove is having engraved for the child, Christine finds the Penmanship Medal in Rhoda's treasure chest. She confronts Rhoda, who initially denies having done anything wrong, but confesses after Christine finds the bloodied shoes Rhoda had beaten Claude with before drowning him. Christine is horrified, but Rhoda doesn't seem to understand what the fuss is about; after all, she says, "it was Claude Daigle who drowned, not me."
While Christine grapples with what to do, Rhoda silences Leroy by locking him in a furnace room and setting it on fire. When she learns what her daughter has done, Christine makes a gut-wrenching decision: she must kill Rhoda to keep her from killing again. She gives her a lethal dose of sleeping pills, hoping she will die without pain, and then commits suicide by shooting herself in the head.
Rhoda survives when a neighbor hears the shot and takes her to the hospital. Nobody is the wiser as to what Rhoda has done, and she is free to kill again.
Patty McCormack portrayed Rhoda in the 1956 film adaptation. The ending was revised to fit the Hays Code, which did not permit characters to get away with their crimes. In this version, Rhoda is killed by a bolt of lightning, while Christine survives her suicide attempt.
She is ranked twelfth on Bloody-Disgusting’s list of "The Top 16 Creepiest Kids in Horror Movie History". Brian Solomon writes, "The grand-mammy of all messed-up horror movie kids, Rhoda Penmark is a pint-sized terror of biblical proportions. If you think the ADHD-addled rugrats you see roaming shopping malls nowadays are bad, you ain’t seen nothin’ yet."
- Brian Solomon, "The Top 16 Creepiest Kids in Horror Movie History," Bloody-Disgusting (May 4, 2009).