It was made into a 1956 movie adapted by John Lee Mahin and directed by Mervyn LeRoy. It stars Nancy Kelly, Patricia McCormack, Henry Jones, Eileen Heckart, Evelyn Varden, William Hopper, Paul Fix and Jesse White.
The movie was nominated for Academy Awards for Best Actress in a Leading Role (Nancy Kelly), Best Actress in a Supporting Role (Eileen Heckart), Best Actress in a Supporting Role (Patricia McCormack) and Best Cinematography, Black-and-White.
Although the novel and play had the mother dying and the evil child surviving, the Hollywood code did not allow for "crime to pay". The ending of the film has it the other way around, then.
The Bad Seed was made for television in 1985, adapted by George Eckstein and directed by Paul Wendkos. It starred Blair Brown, Lynn Redgrave, David Carradine, Carrie Wells, Richard Kiley, Chad Allen and Christa Denton.