Headcanon: Changelings can also feed off other strong emotions like rage/hatred and fear. Consuming either in overly large amounts or making it their only source of food has an almost irreversible effect on a changeling's mind. It becomes incredibly addictive and causes them to become bullies and cruel tricksters early on, eventually leading them to an empty minded madness where they will deliberately inflict pain (or death) upon others to sate their hunger. By this point they cannot be saved.