If you’re willing, I feel there is a bias in your argument that I’d like to explore more with you.
You make the excellent point that plants are living organisms as well, but you also make the assumption that “it is much more humane to kill life forms without a brain.” You then go on to suggest that their sole purpose for existence is nothing more than reproduction.
I’d like to challenge both of those assertions. But before I continue, I want to make certain of my position as anecdotal, as I am not an expert in these matters.
It turns out that plants can see, smell, feel, and have a memory. And according to the scientist in the article, plants and humans also share DNA.
We’ve all heard the advice that we should talk to our plants, as they react more positively (grow) to the sound of our voices. We also know that plants play a vital role in our existence.
As for brains, no they do not have the same type of brain or nervous system that we as humans are accustomed to having. But that is not to say that plants are incapable of making decisions.
Take the Venus Flytrap as an example: it can detect when a bug has landed inside of its mouth, and after having another external stimuli triggered will it decide to trap that bug inside before it devours it for nutrients.
I could go on, such that plants do communicate with other (e.g., grass when cut, fungus creating underground networks to each other).