In episode 12 a old women with in the wall who is a witch, talked to Robin about the history of witches in the Walled City.This is what she said "Witches were the gods on earth created by god. Genetic mutation happened with caused the witches to be humans and mutilated over the years. Soon after Whumans started to label themselves a gods, meaning I think the ruler of this world. They became in jealous and feared witches because of this they burned witches."