According to the Moderator General, they used to be a violent, criminal race. In response to this, the ruling Brains Trust outlawed emotion and the city of Zombos was built high above the "disturbing influences" of the land. In the city, emotion was the only crime and the Zomban people's names were replaced by numbers. The Moderators policed the city and made the people fall in line, while the farmers were ordered off the land they used to love, and replaced by giant farming machines.
Both the civilian Zombans and the Moderators were attacked by Barabara raised by the Zom Emotional People's Organisation rebel Big Hate. The Fourth Doctor stopped this attack by using the harmony booths that removed emotions from the Zombans to create fear and hatred. This caused the Zombans to produce adrenalin, which poisoned the Barabara as they ate the population. When the Doctor left, ZEPO gave the people in the city emotions just like the Doctor's using his e-wave patterns. The Doctor supposed that they would grow out of it. (COMIC: City of the Damned)