That's a great question. I'm going to be writing an article soon that touches on it, about the role of alienation in consciousness.
I take your point that anything with natural inertia might be construed as being "against" everything else just in going about its business. That would seem to trivialize the notion of "anti-nature." The thing is, though, that slaves to nature would find an equilibrium which is, indeed, another natural order. Nature evolves and complexifies by overcoming its older forms. Thus, in some sense, nature is anti-natural just in creating so many new states that replace the old ones. Is natural selection ant-natural in having destroyed the dinosaurs to create mammals?
With humans, though, we have a more deliberate, all-embracing plan based on an opposition to the wilderness as such. We'd be different in acting not just against one natural state but against nature in general, against everything that's pristine in having been untouched by our species. In practice, we humanize, and are therefore against everything that's nonhuman. We have a cancer-like perspective in that we don't settle for an equilibrium with our environment. Indeed, we may not stop until we've destroyed the biosphere.
The articles below go into this further:
