"the gene that encodes for the toxin protein is actually carried by a virus or phage that infects the bacteria" [0]

It seems the advantage for the virus in making the bacteria produce enough toxin to kill a duck fifty times over is that the maggots which eat the dead duck (and carry the bacteria and virus) each pick up enough toxin to kill another duck.

See diagram on carcass-maggot botulism in the reference. (Or not if you haven't eaten breakfast yet).

[0] https://www.nwhc.usgs.gov/publications/field_manual/chapter_...

Not sure how good this is, but: https://microbewiki.kenyon.edu/index.php/Clostridium_botulin...

> but when they are activated and resume growth, the bacteria release several different types of potent neurotoxins that can cause the paralyzing disease botulism. The production of the neurotoxins acts as a defense mechanism for the bacteria for protection from intense heat, increased acidity, and possible fragmentation and damages.

Distracting and compromising the immune system of a large animal while taking advantage of the free resources in the environment would probably have a very strong evolutionary advantage.

Or it could be more like the benefit yeast gets from producing alcohol, which it resists better than most other microbes.

There are bacteria that are poisonous, and there are bacteria that aren't. Clearly, both strategies are viable. So I think any explanation has to explain not just that poison is an advantage, but explain when it is and when it isn't.