@InfraBlue,
The production of toxins (as well as a plethora ... maybe two or three plethori) of other defense mechanisms is a method of avoiding being eaten, not a cause for not wanting to be eaten in the first place.
But you may have put me onto an idea. If a stimulus -- like the presence of a predator -- creates a toxin .... that could be interpreted as "avoiding being eaten" even though, as you said, no higher brain function is involved. (No higher brain is present.)
So, we have the many, many stimuli that could result in death creating many, many corresponding responses; the totality being summed up as "fear of death".
Interesting. I'll have to cogitate on that for a while.