The Bad Seed 1956

The Bad Seed is a 1956 American psychological thriller film directed by Mervyn LeRoy and starring Nancy Kelly, Patty McCormack, Henry Jones and Eileen Heckart.
Kenneth and Christine Penmark dote on their eight-year-old daughter Rhoda. Kenneth leaves on military duty. Monica Breedlove, the Penmarks' neighbor and landlady, visits. Rhoda, pristine and proper in her pinafore dress and blonde pigtails, tells her about a penmanship competition that she lost to her schoolmate Claude Daigle. Rhoda then leaves for her school picnic at the lake.
Christine is having lunch with friends when they hear a radio report that Claude has drowned in the lake. Christine worries that her daughter might be traumatized, but Rhoda is unfazed by the incident and goes about her life. Rhoda's teacher Miss Fern visits Christine, revealing that Rhoda had been seen grabbing at Claude's penmanship medal and was with the boy just prior to his death. She hints that Rhoda might have some connection to Claude's death and adds that Rhoda will not be welcome at the school after the current term ends. Claude's parents barge in, and Mrs. Daigle is distraught and drunk, accusing Miss Fern of withholding information. When Christine finds the medal in Rhoda's room, she demands an explanation. Rhoda tells Christine that Claude gave it to her.
Christine's father Richard visits. Haunted by confusing memories about her own childhood, Christine confronts him and he reveals that she was adopted. Christine is horrified to learn that she is actually the daughter of a notorious serial killer. She worries that her origin is the cause of Rhoda's sociopathy and that her behavior is genetic. Richard tries to convince her that it is nurture, not nature, that primarily influences such behavior.
Christine catches Rhoda trying to dispose of her tap shoes in the household incinerator and realizes that Rhoda must have hit Claude with the shoes, which had left odd crescent-shaped marks on his face and hands that could not be identified. Alternately feigning tears and angrily blaming Claude, Rhoda admits that she killed the boy for his medal and confirms Christine's suspicion that, to acquire a keepsake, she had previously murdered an elderly neighbor when they had lived in Wichita, Kansas. Christine orders Rhoda to burn the shoes in the incinerator.
The next day, the caretaker Leroy teasingly tells Rhoda that he believes that she killed Claude. After Rhoda angrily tells him that she burned her shoes, Leroy opens the incinerator and finds the remains. A drunk Mrs. Daigle returns and tells Christine that she believes that Rhoda knows what happened to her son.
Realizing that Leroy knows the truth, Rhoda sets his excelsior bedding ablaze and locks him in the basement. After some men break open the basement hatch, Leroy runs into the yard aflame, ultimately burning to death. From the window, Christine and Monica see him die, which makes Christine hysterical. That night, a strangely calm Christine etc....