I don’t mean that medicinal plants developed those properties to benefit us, but rather it may have evolved as a response to some animal for some purpose, and it happens to be beneficial to us in some way.

Take caffeine for example, likely it evolved as a defense against pests.

Caffeine at low levels in flowers can stimulate insect activity and even potentially memory, increasing pollination activity.

At higher concentrations yeah it’s an insecticide.

Cool, I didn’t know that. But all the same, the same principle applies.

Possibly too costly. From what I've read, every chemical defense has a metabolic cost. This is why antibiotic resistant bacteria will lose their resistance over many generations if not constantly challenged by the antibiotic.

On the other hand, some plants do kill us.