C. Botulinum has a complicated life cycle that involves infecting vertebrates, killing them, and then infecting the invertebrates that feed on the body during decomposition. The bacteria don't particularly care about the host and just want it dead as quickly as possible so they can get on with their real hosts.

[1] https://doi.org/10.1093/femspd/fty040

I can't answer your question for C Botulinum. But many times we are just in the middle of warfare. That is, it is an accident we are harmed.

For instance, Corynebacterium Diphtheriae is not lethal per se, but then comes a virus that infects the bacteria, and kills the bacteria, and a byproduct of the virus causes the bacteria to cause diphteria. The phage (the bacteria virus) provides no advantage to C diphteriae.

My hypothesis for clostridiums, that includes C botulinum, is that they do want to kill their host, however. They kind of kill the planet they live in as they can thrive in the carcass while it lasts. So, deathly clostridiums have this at least short term advantage over non deathly ones.

C. tetani, perfrigens, botulinum and difficile can all kill you. However, C. perfrigens does it differently, by diggesting you before killing you, "on the fly" or "just in time"

I'm in no way a scientist, but it sounds like the bacteria grows best in oxygen-free, low acidity environments with a lot of protein: that's dead things. Paralyzing your host is a bad idea for diseases that will be permanently destroyed shortly after their host dies, but it's a pretty great plan if it creates a wonderful ecosystem free of pesky immune cells and oxygen for you to grow and produce spores that can happily sit around forever waiting for something to eat them.

The bacteria thrives in rotting meat, so killing the host might be exactly what it needs. It produces lots of spores present everywhere, so transmission from host to host is not an issue.

Edit: what's -> lots

A pathogen's needs will, over time, converge with the needs of its host, but only if the pathogen is only able to survive in one species. When a pathogen is able to feed off of many species, its needs never converge with the needs of any particular host.

It comes in handy for organisms that happen to like feasting on decaying bodies through saprophytic nutrition.

Paralysis enhances the chance of dying.

Being anaerobic limits their expansion.

Thanks to the Great Oxidation Event, cyanobacteria caused.

By the way, often humans and other multicellular Organisms are just battleground or collateral damage in billion year old wars between bacteria and/or viruses.

Clostridium botulinum is anaerobic, and live in soil, so living humans are not a particularly good host. Dead humans could be, though, so natural selection would favor strains that efficiently returned themselves and their descendants back to the soil.

Evolution doesn't need purpose. As long as a feature doesn't reduce the survival rate, it remains in place. The toxicity could just be a side effect of the spores durability and longevity.

Being an anaerobic bacteria and one that can lie dormant for long periods of time, I don't think it needs the host to be alive.

Can it be randomness? Unless paralyzing/killing the host is actively harmful, it might evolve by chance and remain as-is.