The Jacobins killed people in an attempt to purge the public in France. They believed many people were a threat to the nation.
This is actually pretty common and the reason "why" is due to the idea of hysteria.
Hysteria clouds judgement.
There are so many examples of hysteria clouding societies judgement in society.
The Holocaust being one example of how a whole nation was convinced Jews were evil and need to be purged.
Another example happened in America after World War II, when many people feared there were communist spies everywhere in the United States
