Will try to respond to these honestly, with no sugar-coating or euphemistic waffling.
In this naturalistic context, then, there is nothing in the fact of sentience that makes it a preferable form of existence because there is no ground for preferential treatment in this naturalistic context.
That’s true – there is nothing that makes consciousness, even, preferable to non-consciousness. There is only the fact that we are conscious and prefer to remain so; or that we prefer pleasure to pain, life to death.
In such a universe, of course, you do not have to
earn the right for you preference. It is just your preference. You don’t have to say, “I haven’t proven that life is more valuable than death; therefore, I have no right to prefer life.” You are free, in that sense.
then your existence or lack of it makes no difference, then why would murder be impermissible?
We’re living in a world with other people “just like us.” There is a necessary compromise there. For that reason, murder is socially impermissible (obviously, it is not impermissible according to nature, as any natural disaster easily demonstrates). The prohibition against murder is a social prohibition – it’s against the rules. It’s backed up, of course, through a criminal justice system. Our house, our rules.
The reason behind those laws is because the “subjective preferences” of millions of human beings have enough of a common denominator to align, and to create a set-up that “works” to serve those preferences. It does have a “beautiful” logic to it – “beautiful” in my estimation

– that is similar to the intricate grammatical structure of a language. Somehow, society holds together and maintains at least a semblance of order.
A criminal justice system is the bare minimum – the social equivalent of heaven and hell (i.e., if you haven’t a clue as to why you should be moral, the avoidance of hell is always a good place to start

). It is based on the rules that we ourselves have created, based on our preferences (whenever another society, such as the Nazis, change the rules in such a way that is unacceptable to other societies, there is either the application of international law, or there is war).
Aside from this, there are all kinds of admittedly
subjective reasons not to murder. For one, we didn’t evolve to naturally be homicidal maniacs or psychopaths, which means that not all human beings
want to murder. Take the image of a mother nursing her child – that is an image of nurturing, which is also in our nature. Friendship and family relationships are in our nature, as well. We don’t only want violence; we also want peace.
There is also the more intellectual realization that an enemy is a potential friend and ally. Or the realization that he is part of your species. Or the realization – empathy – that he experiences many of the same feelings that you experience.
Whether they believe in God or not, however, there will always be those who don’t buy into the whole social contract idea. They break the rules, and engage in “unacceptable” behavior (strictly speaking, it is not immoral, simply unacceptable to that society). They lose their privileges in that society. Many human beings find such behavior “abhorrent” or “disgusting” or “monstrous.” That is a matter of taste. Whether it is a matter of
natural taste is besides the point, but I do believe that it may be – social instincts as a product of evolution. Even criminals abhor those who abuse children, for example.
No “ought” could exist other than to promote a merely self-preservationist pragmatism. If we both want to survive, we should not kill each other, but what if I feel my survival is better promoted by doing away with you? There cannot be anything “morally” wrong in killing you because you have no inherent “value,” in fact, no one does. It might not be pragmatically feasible to kill others, but it also could very well be in some situations. Your strictly naturalistic context would necessarily be silent on such a question.
Again – it’s against the rules. That’s what wrong with it; it’s wrong because human beings (more numerous than the criminal minority)
say that it’s wrong. It’s socially forbidden. We ultimately don’t have to prove that preference – neither to the universe, nor to the naysayer. It is democratic, in that sense – the prohibition against murder, stealing, or rape. What it’s grounded on, ultimately, is the preference for life over death, physical well-being over physical suffering, and the recognition of the kind of society that promotes those goals. We are free to have made those rules, and free to make life difficult for those that break them.
But you have the same problem even with an absolute morality. There will always be those who don’t subscribe to it and don’t believe in it. You
still need jails for those individuals; and, as a dissuasion (which doesn’t always work, either) you have the threat of hell and of God’s punishment. But you will have those who reject God’s law, of course – just as you will always have those who reject social norms.