A movie about a 12-year-old homicidal maniac almost has to become a cult item, but Mervyn LeRoy’s 1956 production seems to have done so in spite of itself. Overlong, stiff, and about as suspenseful as a detergent commercial, The Bad Seed has one small asset, Patty McCormack as the child, but that’s about it. With Nancy Kelly, Henry Jones, and Eileen Heckart. 129 min.