At first glance it may seem silly to assert that you can’t believe murder is wrong unless you believe in God. But that’s not the right question. If we are interrogating the foundations of morality then we must ask why you should believe murder is wrong strictly on materialism. Does a morally binding prohibition against murder follow from the propositions of materialism?
If I’m a materialist I certainly can believe humans have intrinsic value and that therefore murder is wrong. But can’t I just as easily believe that we are insignificant specs in the cosmos—on materialism can’t I validly believe that humans are simply congeries of particles, forces, electrochemical interactions, etc. that happen to be configured in a certain way just like everything else? Is there anything in materialism that tells me why I should place a higher value on molecules arranged in a human form than I do on clouds of hydrogen molecules floating in space?
It is worth noting that Christians have no real problem here. Christian anthropology teaches that humans have value because we are made in the image of God, and we are given an elevated place in God’s creation. Also, God explicitly commands us to love one another, not to murder each other, etc. Also, we are such special objects of God’s love that he sent his only begotten son into the world to pay the price for our sins.
So it seems that Christians can point to someone who murders in the name of Christ and say, “He is a bad Christian” in a way that materialists can’t point to a Stalin or a Mao and say, “He is a bad materialist.” Is there anything in the actions of Stalin or Mao that is inconsistent with materialism?
Certainly they may be inconsistent with certain materialist moralizers, but then what in materialism as such enjoins us to choose those moralizers over Stalin or Mao?
If the universe is ultimately indifferent to human values, and if when we die our conscious selves are completely and irreversibly obliterated, and if there is no final judgement, then in what sense are we obliged to follow the principles of a given materialist morality?
So again, the question isn’t whether you can be against murder if you are a materialist. The question is why anyone ought to be against murder on strict materialism, when Mao and Stalin remain materialists in good standing. And if there is no reason you ought to, then a prohibition against murder is ultimately a subjective expression, and therefore not a moral prohibition at all.