Fritz and his brother, Christian, had a grandfather who suffered from a degenerative mental disease and was put into a psychiatric hospital until his death in 1943. The disease ended up being hereditary and was passed down to both brothers and possibly their father. Where as it took till adulthood for Fritz to start developing the disease, his brother wasn't so lucky. Christian started showing signs from a young age and is implied to have killed their father.
Ennio Morricone - Spasmo (1974)
Fritz is revealed to be behind the murdered mannequins.
Fritz is revealed to also suffer from the same disease as his brother.
Much to his dismay.
"Your brother, Christian's disease is hereditary."