When it gets down to it, the "nature" faction in the movie doesn't really have a real argument. They don't want humans to cut down any trees, so they kill the humans. The humans weren't even really killing animals, they were just fighting them off so they could get at the iron ore. The forest is important because it is intrinsically valued, not because of any actual traits that can be empathized with, or because of an argument about future sustainability. The fact that the inhabitants of the forest are sentient certainly changes the dynamic, but they were the ones who attacked, and it is the humans who were forced to defend themselves.