Dawkins says “justice is a human construct of great importance in human affairs.” And Dawkins believes that there is probably a Darwinian explanation that explains justice. So, our concept of justice is just a convenient Darwinian happenstance. I believe he says “blessed precious mistake” in his book The God Delusion. Of course, Nietzsche would disagree. Nietzsche in On the Genealogy of Morals doesn’t think it’s blessed or precious.
Also, if justice is merely a human construct then the cannibal clan in Cormac McCarthy’s book The Road are not wrong in keeping people locked up in the cellar in order to slaughter and eat.