Are the robbed people idiots? If you kill a witch while they still have regrets it spawns a calamity. If anything you should be attempting to help all the witches get rid of their regrets.
I'm thinking the situation could be a mix of people's distrust in anything a witch might say (the claim or regret while dying spawning a calamity might be the foe's attempt to weasel out of being killed in their eyes), and witch hunts, where beastmen were murdered and nothing would happen (so the ignoramus would point at those and say 'but nothing happened then!', then claim the warning was a bold faced lie).
Wouldn't surprise me if they got the cause of the post-mortem calamity backwards at some point, which they firmly believe in.
They have a list of witches they've killed (either as that very group, or the organization as a whole), so most (if not all) of the victims beforehand could be beastmen.
That, or there's some religious/overconfident lot that think they can prevent calamities from happening after a witch's death.
Or want the world to end, so they incite the forces from the background.
So the witch's name is something like Nina-niina? Neat.