Though we understand that everything is natural, from fighting of man and beasts, because of the law of the survival of the fittest, to “what goes up must come down” because of the law of gravitation, there are some things that we feel unnatural about.
Just look deep into yourself honestly. For example, is your sexual drive always natural? Do you always have to be driven by it? Is your backstabbing of your colleague natural? Do your animal instincts justify you to be animal and inhumane? Though man is of the animal kingdom (as classified by man), is he to behave like an animal? Or is there a conscience inside that knows exactly what is right and wrong?
Which side of your nature do you feel the closest to, being the most comfortable with? I suspect it is your Buddha Nature!