This is an argument, though crudely put in this case, I've never had a good answer to. There have been countless attempts in history to define some sort of morality that is not based on a man in the sky telling us what is right and wrong. Jesus is some people's man in the sky, but he made famous the rather durable code ("do unto others as you would have done unto you") that doesn't seem to require a divine sanction to make sense. Kant referenced a categorical imperative that seems to my crude philosophical mind to be a generalized case of Jesus' Golden Rule: "do as you would have others in your position do." Most recently, Sam Harris played the semantic trick of re-defining good as that which benefits the welfare of conscious creatures, with the Kantian caveat that conscious creatures should be seen as ends and not means to an end.
The efforts of Jesus, Kant, and Harris all seem to make sense to me. But where these systems all seem to fail as moralities is that, without any objective creature to tell us so, these moralities still rest on the preferences and intuitions of the hearer, giving them a rather circular quality. In other words, if I choose one of these moralities upon hearing it but later run up against circumstances that change my sense of the greater good, should I trust these old moral laws or my new intuition? Always choosing ones' intuition seems a kind of solipsism while always choosing a received moral system seems a kind of fanaticism.
Moreover, all these secular values seem descriptive of how a good human citizen acts in a society we'd all like to live in rather than a description of an objective truth outside our collective experience. This may seem like splitting hairs, but what do we do when we encounter secular moral systems that don't make sense to us? Imagine a person who held that scientific advancement was more important than furthering of the welfare of conscious creatures. How do we weigh this system against Sam Harris', except to say "I like the implications of one but I don't like the implications of the other?" I haven't found a way around that question either.
Theists may chortle at this existential quandary atheists find themselves in, but the problem is no better for theists. After all, why should a god's commands be moral? The way the Judeo-Christian God is able to prove his edicts to be moral to us is that the moral go to Heaven while the immoral go to Hell. In modern sophisticated religions, Hell has been defined down from a visceral place of tortures to an eternal cutting off from the bliss of being with God or simply to Nothing. Nevertheless it still boils down to a principle of "be good or you'll feel it in the end." Under the old tribal God, might makes right. Under the modern God, morality is like a healthy diet. What's striking is that we're still relying on utilitarian principles and rational self interest, albeit with the scales weighted by fantastical stories of the afterlife.
A lifetime of thinking on my part adds up to this: there is no such thing as morality. Even if there were a God there would be no such thing as morality. If a person has a belief about the right thing to do when she is in possession of all the right facts, there is no "proving" that person wrong. We can appeal to the intuitions of others and hope everyone else agrees with our informed intuition, but there's nothing we can reference outside of the effects of the principles we are espousing.
This is not to say that all beliefs are created equal. When Christians say that they oppose homosexuality or divorce based on God-derived principles, I can appeal to the truth, as I see it, that there is no personal God whose thoughts they could possibly know. Now, it is true that the truth is always up for debate, but there at least we are appealing to something outside ourselves. We do not have to take the path of what is commonly referred to as "moral relativism" where nonsensical barbarity is excused because we can't judge other cultures. We can judge beliefs not just based on our own intuitions, but on our evaluation of the basic truths that underly those beliefs. This means that not everyone's belief is equal if we assume that there is an objective reality. (I realize that some people dispute this, but I'm less troubled by that assumption.) The propagandist thinks that truth has a debt to pay to morality, but it is the opposite. Any so- called morality we espouse must be relative, but the truth is absolute.
I guess, then, we don't really have a universal answer for the hypothetical killer rapist in the Duck Dynasty story. Whether we believe in God in not, we'll still have to express an assertion based on our own view of the world. That might not be satisfying to those that would like the perfect retort to all the madness in the world, but it's all we have, and it is the truth.