Can we refer to nature as the answer to killing? As the Buddha observed, life, ageing, sickness and death are the 4 noble truths. Life - every species want to survive but because of scarce resources we have survival of the fittest which sadly involves killing. Ageing - every species know that they cannot live forever and reproduce to pass down their genes before Sickness and Death catch up with them. If one does not wish to continue this cycle of birth and death, killing should be eliminated. If we talk about the Yin-Yang balance of life and death to ensure the survival of most, or if not all, species, then killing is not that "bad" or "good" but just the antithesis of living, all part of Tao (universe).