While I get your point, poison is not so simple as that.
There are hemotoxins, that destroy the blood(which is why it was so effective on the blood cultists), neurotoxins that paralyze the body by blocking neurotransmitters, necrotizing toxins, and all sorts of toxins
There's even a toxin that:
- Causes pain so excruciating that patients beg doctors to end it all
- And causes "feelings of impending doom" so severe patients lose all hope, hence ending it
- Not fully lethal, so you can live through all that hell
I frankly don't know how the Irukandji jellyfish managed to evolve a toxin that causes
feelings of doom, and honestly I don't want to know
Well, it is in Australia.