It's not about being pure. It's not about keeping your hands clean or avoiding guilt. Imagine birds living in a forest. Humans come and cut the forest down and build barns and plant crops. If some birds are able to live in the barns, or eat the crops, they don't say, "I'm not going to live in the barn -- that's cheating," or "I'm not going to eat the crops, because then I'm just part of the system." Of all the species on Earth, only humans are that stupid.