So it must be then that the claim, Human life is sacred is justified. There are two general types of justification for the claim. The argument for the sacredness of human life from the claim that humans are ensouled is generally a non-secular argument. A soul is a divine thing, divine things are sacred so ensouled things are sacred to. What is divine is sacred.
Human beings have souls.
Thus, human life is sacred. If it's the mere fact of having a soul, then presumably, if chickens were ensouled, they'd be sacred too. That said, however, many secularists who believe that life is sacred because the soul is, believe that, in fact, human beings are the only things with souls. Thus, X's life is sacred. Thus, X's life is sacred.