Generally there are two possibities for attributing value. Either people are worth something because of the physical components they possess or there is something intrinsically valuable about people that would transcend the physical. The former is the natural conclusion to the atheistic worldview and the latter implies that there is something beyond simply naturalism and flows from a theistic worldview.
In our Declaration of Independence, it seems fairly clear that the founders thought that people were intrinisicly valuable as they wrote, "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights" They may have been wrong, but the way that we begin influences the way we think in other areas.
If man is simply the result of random chance, then there is nothing that makes him valuable apart from possibly complexity. But even then to kill an animal or to kill a man really should not be seen as too far different in regards to destroying something of worth. In fact, if morals are relative, there is nothing wrong at all, maybe it could even be used for population control much the way we control deer.
Reading that should strike a nerve, but ask yourself, "why does that strike a nerve?" In my estimation, it is because we recognize that people are intrinsically valuable - that is the blank slate position that we start with and I think we have to be talked into something else.
Are there people who disagree? Yes, in order to be consistent, they must. Consider this interview with one of the most consistent people on the naturalistic side I am aware of, Peter Singer:
"Question: What about parents conceiving and giving birth to a child specifically to kill him, take his organs, and transplant them into their ill older children? Mr. Singer: "It's difficult to warm to parents who can take such a detached view, [but] they're not doing something really wrong in itself." Is there anything wrong with a society in which children are bred for spare parts on a massive scale? "No." " -World Magazine Nov 27, 2004.
Notice, he is not talking about stem cells; this is a baby that has been born specifically for this purpose. Singer's views are not secret, he is well known for his thoughts on infanticide.
So, where do you stand? When you look at your children are you looking at nothing more than a bundle of cells that could legitimately be treated as a resource and even exploited? Or are our children intrinsically valuable? Are we intrinsically valuable? It is my position that we are and it is also my position that the normal experience of man is that we are, this is why we look at a guy like Martin Luther King and say that he was a good guy and we look at Hitler and say he was evil.
For those just joining, I am trying to show that it is intellectually reasonable to believe in theism. The first topic was a chance for atheists to defend their case positively and the second was the sense of moral motions that we experience being grounded in a being that is both infinite and personal. These arguments are not meant to be slam dunk deductive arguments, but instead look at our experience and ask which way does the evidence point. Atheists, at least at this point, have been able to offer no solid rationale for holding to atheism aside from presuppositionally or as the default position. So these arguments are meant to say that atheism does not have the authority to be the default position since it is not what lines up with human experience. These are arguments from inference. It is recogonized that some just disagree - that is fine. At the very least, I would hope that those folks would be able to walk away thinking I was mistaken but recognizing that theism is within the grounds of rationality.