You made no mistake, that actually is what happens to ghouls, it is not pre-determined for them however, it depends on many factors, like environment, social interaction, rate of brain decay, etc. but it is generally acknowledged that ghouls will eventually turn into feral ghouls, hence the predigest towards them. Human consider ghouls a liability and fear, in their ignorance, that they will become feral at the drop of a hat.