That depends on the variety. “Sweet” cassava is quite simple to prepare, and can be boiled like potatoes. It is one on my go-to foods in the Dominican Republic. But “bitter” cassava – closer to the wild type – has a much lengthier and more elaborate process involving grating and leaching.
Why doesn’t everyone just grow “sweet” cassava and save the trouble of prparing the “bitter” kind? Well, the “bitter” kind is more resistant to insects, and hence less likely to experience crop loss in an insect outbreak.
And yet it is a staple food in much of the world. What it provides is bulk calories, which are needed where people live a strenuous, calorie-intense livestyle; but with its bland flavor, it encourages adding relishes, garnishes, and side dishes which would provide the other needed nutrients. Remember, though, that in evolutionary terms, the cassava plant stored calories in its roots for its own later use, not with a view toward what humans would need.
How about this thinking exercise (theory). Solanaceae developed the basic alkaloids in order to cause mammals that dare to chomp on it. Depending on the species, the bouquet of these chemicals will confuse, or induce sleep, or madness, or sedate, or dilate pupils in a mammalian, causing it to stop moving, making it die and fertilize the growing environment of the solanaceae.
You make it sound like the plants have a specific agenda and conspire against other organisms/nature. That isn’t how evolution works at all. I don’t see why we’d operate under other assumptions.
Populations of plants don’t decide to do something and then follow through with that decision. A desire did not cause the change. All we know is all we know, the rest is a just-so story and could very well be fiction. What is the value in creating a potentially fictional story? All we know is that alkaloids prevent many herbivores from eating the plants that contain them and, therefore, it is plausible that plants contain alkaloids today because it improved the reproductive success of their ancestors. Why not just leave it at that? The fertilizing with dead bodies part is unnecessary icing on the unnecessary cake–and almost certainly not correct as dead bodies don’t drop and fertilize the individual plant that possessed the trait that killed the animal.
If there’s a population consisting of many plants of the same species, some of which produce alkaloids and some of which do not, and given that herbivores are less likely to damage plants with alkaloids, then those individuals producing alkaloids will leave more copies of their genes to the next generation (on average). Imposing a desire or motive upon the plants requires that you propose a biological mechanism by which that desire or motive is manifest.