Hi this movie is old it goes like this: a anonymous woman leaves a golden haired child on a family door step and they take her. The child starts to kill off the other children but the mom won't believe the husband. The mom sees the golden haired child push her daughter out of a house window to here death. Then all I remember is in the end the mom and the husband see another golden haired child with another family walking down the street.

The Godsend (1980) (http://www.imdb.com/title/tt0080802/)