I remember once accidentally videotaping the ending of this flick and never found out what it was called. It may have been a TV movie and the latest date would be 1981 (the year I bought my first video machine).
This is how it climaxes.
The mother of a cute-looking blond boy of about 11 is alone at the top of a house which is alongside a cliff top with a sheer drop to the rocks below.
She is absolutely terrified as her son seems to have gone mad and is slowly climbing the stairs brandishing a knife. He seems to be getting revenge for something (the death of his father?) and keeps wailing "I'm gonna get you momma!" as he comes closer and closer.
As I recall the mother has to destroy her son and he ends up on the rocks below, as had the boy's father due to an accident some years before.