I saw this movie on TV 20 years ago.
A mad woman trades a baby in his cradle with her own newborn. She is charged by police for murder and imprisoned in a psychiatric hospital. The newborn scratches with his "mother's" face with his nails.
The newborn is a boy, and is silent, sad and shy. He is bullied by some students, and an unidentified killer kills one of the bully boys with a shotgun shot to the head. The killer then hides in the attic of the silent boy's home.
The mother of the boy discovers he is not her real son and investigates to discover that the real mother of her "son" is a murderer and her biological son is in a orphanage. When she returns to her home and discovers the killer of the bully, who is also the father of the boy and husband of mad killer woman, at her house. He tries to kill her but the silent boy defends his adoptive mother and shoots the man who is actually his father.
The biological son and the adopted son live together happily in the new family with mother and father.
This likely came out between 1989-93, it was probably a made-for-TV movie from the USA.