No! Those toxic chemicals are indeed natural.
I'm going to challenge you a little bit because I don't think your description helps explain what nature is (the theme of this thread). You are right to say Theaetetus' definition doesn't contribute since he relies on the term "nature" for his explanation and then says what is not
nature instead of what is. Likewise, Nameless says nature equals existence; but since everything we know exists, then everything is natural and so why have a term such as natural when the term existence should suffice?
I suggested we use emergent conditions as an objective
standard for defining nature because we tend to self-centrically describe nature merely in relation to our own existence. But to be objective it seems we'd try to see what is it about reality that gives things what we humans term a "nature." I'll give examples of how to use emergence as the standard.
Emergence describes how things we know exist (we "know" because we can observe them) came from prior conditions
. For instance, life showed up on Earth some 4 billion years ago. Before that we assume there was no life, so what conditions
caused the first emergence of life? Before Earth came into existence, there were only conditions that would allow Earth to appear (i.e., emerge). What were those conditions
? Before that the universe appeared, and before that the Big Bang occurred . . . what were the pre-existing conditions
that caused each to emerge where they did not exist before?
Now, pre-existent emergent conditions are only part of the story for us ("us" meaning, users of the term "nature"). We also are concerned (especially recently) about how dependent we still are on emergent conditions. The Big Bang no longer is relevant to us, nor is what created Earth, because those emergent factors are beyond our influence. However, what caused life to emerge on Earth is very much subject to our influence, and because we are life (or are alive at least), now life's emergent conditions, and the degree we still are dependent on them, have become important to us.
You argued that we should accept "toxic chemicals are indeed natural" because:
. . . we as humans need to stop looking at ourselves as detached from, and independent of, nature.
But if we look at it that way, we have eliminated any meaning for the term "nature." So if nature is to have a meaning, I suggest seeing nature as describing a situation where: the nature of a thing is the conditions which allow it to exist now, and that those conditions are inextricably tied to what caused it to emerge in the first place.