It wasn't long after a municipal worker in the Tehran neighborhood of Ekbatan found hands and other human body parts in a trash bin on May 16 that police identified the remains. The victim was Babak Khorramdin, a little-known 47-year-old filmmaker and university professor. The grim story that has since unfolded chapter by chapter has made headlines and shocked Iranians across the nation.

Upon questioning by police, the victim’s parents confessed they had killed their son by...