How Can Objective Morality Come From A Subjective Source?

In response, a Secular Humanist might reply with the words of the Humanist Manifesto III, “We are committed to treating each person as having inherent worth and dignity.” However, which value takes priority when two values come in conflict? In cases where one humanist ideal is weighed against another, how is the priority to be determined? Is human dignity more valuable than the “good life” for the mass population, or is it the other way around? Is an individual’s value higher than the greater good of Humanity? When “ethics are derived from human need,” there is no way to determine which is more important. 
If one can justify that murdering his neighbor is better for the common good, then why shouldn’t he murder his neighbor? Humanism has no satisfying answer for this, other than to give an unsupported assertion that humans have worth and dignity.⁠1 Where does that worth and dignity come from; the evaluation of other humans? What if a separate group of humans evaluates that worth differently? Who wins?
Although secular humanists operate as if there is an object morality, there is no explanation from within the ideology to explain where that objectivity comes from other than the subjective claim of “human needs” and “interests.” There is nothing in the Humanist Manifestos that truly restricts cruel behavior on a absolute basis. The Humanist Manifestos forbid cruelty much like a lazy parent might with statements equivalent to, “because I said so.” 
This leaves secular humanist morality irreparably moribund. It should be noted what is not meant by this description. This is an argument that points to the source of objective morality, not the implementation of said morality. Obviously, secular humanist are capable of being moral. However, so are humans of all kinds. One does not need to be a secular humanist to be moral, so once again there seems to be no advantage, philosophically, logically, or morally, to being a secular humanist.
On the subject of objective morality, C.S. Lewis said, “human beings… have this curious idea that they ought to behave in a certain way… Secondly, that they do not, in fact, behave in that way.”⁠2 He goes on to point out that this curious habit of humankind reveals that all humans have an inherent understanding of the law of nature. He uses the term to mean what we might call universal morality. His conclusion is that this law has the markings of the absolute and universal, rather  than the arbitrary. 
The Secular Humanist must claim that, however, this moral law is arbitrarily in its origin. He may operate as a moral objectivist, but the Secular Humanist has no satisfactory answer for how morality could be absolute in its operation but arbitrarily in its origin.

1 Ibid.

2 Lewis, Clive Staples. Mere Christianity. Zondervan, 2001. 8.