The poison was made to mimic an efficient virus, to spread illness rather than kill the host fast. It made the host aggressive and emotionally indifferent to others, enough to make them a sort of social poison machine. A few drops here and there and the decay set in, a sort of race to the bottom, survival of the nastiest.
The poison targeted the prefrontal cortex, taking out their self control, empathy and logic. They became a sort of retarded Dalek, a primitive monster. If the criminals found someone inconvenient they shot them, but if it was a vendetta then they used the poison. Even a saint would turn into a demonic predator once it took effect; the newspaper stories were of folks tearing apart their own families over petty grievances, mostly imaginary. It was the poison we all feared, the stuff living nightmares are made of.
It was the ultimate in state control, remote control poison capsules. If you displeased the state you were terminated.
When a city became unproductive and the officials decided the residents cost more to keep than they were worth, they let time limited poison producing bacteria into the air, water and food. They were programmed to die in a few days in a similar way to a radio half-life, leaving the land clean, aside from the corpses of all ages rotting and pecked at by flocks of crows.