Now we ask him 'Why shouldn't we kill?' This is a pretty ambiguous question, and we could be asking a clearly normative question to which the answer might be 'because there's an injunction to the effect that you shouldn't'.
The answer to this question actually depends on whether you are doing normative ethics, or talking about morality. In the former case, a sensible answer would be: "because, as a matter of fact, most individuals and societies agree that "non-killing" is a morally relevant 'value', where 'value' means a conative ambition (i.e. what "should" we do?). As a normative ethicist, I fall back on such widely-shared values".
When doing morality in a sort of common-sense way, the answer is more complicated. Generally speaking, you're going to find that such 'values' (or, again, conative ambitions of the "should" variety) are a part of the "moral core" of individuals, what they take their "morality" to be about. This moral core is influenced by many factors, including their biology (so, yes, they're generally going to share most other humans' values), society, perceived moral authorities, etc. It can also be influenced by ethical debates they take part in: most people can be convinced that they should drop some moral values and take up others.
All of this means that the real world is quite complicated, and does not fully reflect any of the "moral positions" that philosophers like to talk about.
All of this means that the real world is quite complicated, and does not fully reflect any of the "moral positions" that philosophers like to talk about.
That is doubtlessly true, though I wonder if its an entirely fair criterion. While most ethicists would agree that the right view should reflect actual everyday moral judgements, nothing in particular holds them to that. It's simply possible that no one is presently good, and that the everyday moral judgement people make are terribly corrupt and over-complicated compared to the correct judgements.
I think there’s a confusion in our discussions of deontology and consequentialism. I’m writing this post to try to clear up that confusion. First let me say that this post is not about any territorial facts. The issue here is how we use the philosophical terms of art ‘consequentialism’ and ‘deontology’.
The confusion is often stated thusly: “deontological theories are full of injunctions like ‘do not kill’, but they generally provide no (or no interesting) explanations for these injunctions.” There is of course an equivalently confused, though much less common, complaint about consequentialism.
This is confused because the term ‘deontology’ in philosophical jargon picks out a normative ethical theory, while the question ‘how do we know that it is wrong to kill?’ is not a normative but a meta-ethical question. Similarly, consequentialism contains in itself no explanation for why pleasure or utility are morally good, or why consequences should matter to morality at all. Nor does consequentialism/deontology make any claims about how we know moral facts (if there are any). That is also a meta-ethical question.
Some consequentialists and deontologists are also moral realists. Some are not. Some believe in divine commands, some are hedonists. Consequentialists and deontologists in practice always also subscribe to some meta-ethical theory which purports to explain the value of consequences or the source of injunctions. But consequentialism and deontology as such do not. In order to avoid strawmaning either the consequentialist or the deontologist, it’s important to either discuss the comprehensive views of particular ethicists, or to carefully leave aside meta-ethical issues.
This Stanford Encyclopedia of Philosophy article provides a helpful overview of the issues in the consequentialist-deontologist debate, and is careful to distinguish between ethical and meta-ethical concerns.
SEP article on Deontology