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.
If we are merely evolved from animals then why shouldn’t we act like animals? If we are brutes, why shouldn’t we be brutal? (Nietzsche, for instance, promotes the strong acting like “large birds of prey” and freely abusing the weak “lambs” because after all that’s what comes naturally and there is no God to impose an objective morality).
Also, Dawkins doesn’t and can’t believe in final justice. And Dawkins, who ironically often brings up the problem of evil, can’t believe in evil. He says things are unjust but injustice is merely socially constructed from a Darwinian perspective. So, if certain communities do certain things that we personally don’t like—such as keep people locked up in their basement to eat later—they are not actually wrong. We just see things differently based on our genetics and the community in which we find ourselves.
Can Darwin and Dawkins produce moral duty? Can people be good without God? Actually, that would be an unhelpful question if there were no God. Because if there were no God there would be no ultimate good.
However, if we ask: “Can people do good without God relative to their own subjective whims?” The answer would be: “Yes. People can do the “good” that is right in their own eyes. They can pillage and plunder. They can lock people up in their cellar.”
If there is no God, if justice and morality are merely a human construct, then whoever reigns, reigns; whoever is oppressed, is oppressed. There is no moral duty. There is only power and subjection.
Richard Wurmbrand who experienced ghastly torture said,
“The cruelty of atheism is hard to believe. When a man has no faith in the reward of good or the punishment of evil, there is no reason to be human. There is no restraint from the depths of evil which is in man. The communist torturers often said, ‘There is no God, no hereafter, no punishment for evil. We can do what we wish.’ I heard one torturer say, ‘I thank God, in whom I do not believe, that I lived to this hour when I can express all the evil in my heart.’ He expressed it in unbelievable brutality and torture inflicted on prisoners.”
Thankfully there are many people that don’t believe in God who are considered “good people.” However, they have no ultimate moral duty to be good. Dawkins would say here that people can be good without some disciplinarian in the sky threatening to swat them every time they get out of line and do something bad. I agree. People can do good things even if they don’t believe in God (This is called common grace and Darwin and Dawkins have both been the recipients of it. Even their work in science, the knowledge that they have shown, and the contributions that they have made demonstrate common grace). Yet, for the atheist, moral evil, ultimate good and bad, cannot actually exist.
So, the problem is not just can we be good without God, but is there good without God? And it seems the answer has to be no. No there is no good and there is no bad (evil) without God. Yet, Dawkins and others seem to repeatedly contradict this truth because they bring up the injustices of religion all the time.
 See the debate “Lennox Vs. Dawkins Debate – Has Science Buried God?”
 Nietzsche said, “I expressly want to place on record that at the time when mankind felt no shame towards its cruelty, life on earth was more cheerful than it is today,… The heavens darkened over man in direct proportion to the increase in his feeling shame at being man” (See par. 7 of the Second Essay in On the Genealogy of Morality).
 Richard Wurmbrand, Tortured for Christ, 38.