SPOILER: Plot based on horrendous child abuse
SPOILER:
What makes CASTLE FREAK so terrible is that the plot is based on horrendous child abuse. The duchess had a child out of wedlock by a common American soldier during the ending days of World War II. In that European culture, so many cultural and social taboos were broken. A young woman of aristocratic heritage has illicit sex with an American soldier. Not only was the man a foreigner-American-he was a common enlisted soldier, not even a commissioned officer. And then after the war the soldier returned to the U.S.
The movie is not clear whether the duchess was enraged at the American soldier leaving her or not, but it is clear that whatever the cause, the duchess was unbalanced. The duchess essentially CREATED the monster. She deprived her son of a normal, human upbringing so that his mind was little better than that of an animal. She also disfigured her son. It was never shown or explained but there is a quick frontal scene of the always-nude monster son where it shows his penis was severed a lot time ago, cut by a sharp instrument and not torn off. I don't know for sure if the abused son was missing his tongue but I think the mother may have removed it, too.
The real monster was the evil duchess. The monster, the abused, disfigured son, was as much a victim as he was the killer.