It is a philosophical fact that morality is an inherently religious concept. Whereas science describes what is, religion describes what should be. Science can tell you, in great detail, what goes on during a murder, it cannot tell us if murder is wrong, or why.
Contrary to popular belief, Evolution is also a religion. It is not science, as it is not a description of reality based on repeatable observation. In fact, whether true or false, Evolution could never be scientifically demonstrated. This is because it is a claim about history. Specifically, it is a claim about origins. How did we get to where we are right now?
The answers to those questions vary between Evolutionists, but there is a general consensus on a few things. One thing most Evolutionists agree on is that all living things share a common ancestor. Humans are typically said to have either descended from apes, or share a common ancestor with apes. Going further back, all mammals can allegedly trace their ancestry back to one mammal. Further back, we are all ultimately descended from a single celled organism, which magically created itself somehow. Of course, there are also theories about how that was directly created by an alien race, but let's not complicate things unnecessarily.
This ultimately means that, according to Evolution, human beings are directly related to trees. We are very very distant cousins. And yet, most of us see no issue with cutting down trees. We make things out of them. Chairs, beds, doors, walking sticks, spoons, not even the most vocal "tree huggers" could get anywhere in life without someone, somewhere, using a wooden product.
So, here's the question: Why is it ok to cut down our cousins, the trees, but not ok to cut down our brethren, our fellow humans? For a Christian, the answer is simple. Human beings are not related to trees. We are special creations, made in the image of God, and He has commanded us not to murder. But for an Evolutionist, the answer is not so simple. They can describe what happens in murder. They can explain the psychological reasons why most people are opposed to murder. They can even make up a wide range of weird and wonderful stories about how we came to believe murder is wrong. But there is no possible way, if Evolution is true, to say that murder is objectively wrong.
All human beings have natural moral instincts. Christianity explains where those come from (Romans 2:12-16), as well as objectively justifies which of those moral instincts should be followed and why. Evolution, by contrast, is just a fairy tale designed to explain our reality so that believers do not need to acknowledge their accountability to God.
Nevertheless, denying that accountability does not erase it. A time will come in which every Evolutionist will stand before God in judgement, and every sin, whether it be murder or even an idle word, will be weighed. The question is, do you want to stand before God with your own sins, or with Christ's righteousness? There's only one logical answer.