Frailty (2001)

FBI agent Wesley Doyle is startled by the spontaneous declaration of youngster Fenton Meiks about how his father's delusions to have a divine mission as avenging angel required him and his brother Adam to become his 'demon-slaying' murder accomplices. And how hard an dangerously revolting proved. But when Doyle accepts to be shown concealed victim graves, the plot twists in the present just as gruesomely.