It is not inconceivable that a species might exist that would accept murder (or something like that) as part of their daily lives (though it would likely need to exist under circumstances that would not be harmful or would even be beneficial for the survival of that species). Who are we to say it's wrong if every single individual of that species thinks it's right and it serves their survival.
We are not such a species and to kill another human is almost universally considered unacceptable and harmful by humans (for practical good reasons in my opinion). It is also a matter of how such things make us feel. Even if feelings are arbitrary, they are a property we possess and they are closely related to our conscious experience. They are also what makes us individuals and in my opinion we should proudly embrace our individuality, because what else is life and consciousness besides embracing ones own complexities and experience (which to some degree includes that which we call the external world)?
I almost always value the autonomy of any creature over their own lives above anything else. Some people possess values that may conflict mine. When it comes down to the basics, there is not much more to it. A particular value in vacuum is not irrational as such, but often poor reasons are given. Rationalization in general is not difficult and people love to do it, but what is less often discussed is that reasons aren't necessarily needed. Having them and thinking that's all there is might in fact make things worse. People try to rationalize arbitrary emotional values when in fact doing so and being ignorant of the arbitrary emotional basis is a sign of irrationality. Rationalization matters only to the extent that conflicting values are held. Sometimes, however, two people might have conflicting values without holding any conflicting values of their own. One might think this would be the only reason for war. To defend ones way of life against someone whose highest priority is to eliminate your way of life and replace it with something you can never accept. However, from what I know that has never been the primary reason for war. Go figure.
Values become interesting (or even problematic) only after you find the continued existence of your way of life is contributing harm to others or in general you find that your way of life is in conflict with your values. You are forced to weigh the value of your particular experience relative to how much you value that of others. You can try to do balancing, give up something that you don't value that much, but eventually you might reach funny conclusions such as that your particular experience or even existence isn't justified even on the scale of your own species, let alone on some cosmic scale. Then again, you are too small to know this to be true, but as you have nothing but your own faculties of reasoning to arrive at a conclusion, you are forced to arrive at one anyway. Unfortunately after you follow all the lines of reasoning you might still find it's all irrelevant and arbitrary. Sometimes all that matters is that which feels right. Thank goodness we appear to have something resembling the idea of conscience and intrinsic will to live, at least some if not most of us.