I'm going to propose a rationale for why we believe murder is bad. I am convinced that one can define the "badness" of murder...
I'm going to propose a rationale for why we believe murder is bad. I am convinced that one can define the "badness" of murder by appealing only to what we know about how humans evolved and how humans interact. I don't think anyone has demonstrated this claim scientifically yet, and I am not the one qualified to conduct those studies. I know there's a body of work on evolutionary theories of morality, but I've had little chance to read much of it. Still, the hypothesis is, I think, reasonable, and until it is falsified seems quite sufficient (to me, at least) to form an ethical foundation. The underlying goal here is to suggest that all morality is really driven by socialized, evolved instinct. I'm starting here with a fairly plain example just to try to keep things simple.
Let's define murder, for the sake of this discussion, as the intentional killing of a human being. One may quibble about some of the boundary conditions - such as what exactly is a human being, or what constitutes intent - but these will not affect the core definition.
In a population where murder is common, individuals will feel highly stressed. This stress lowers the overall energy an individual can devote to reproduction. Entities that do not fear the risks associated with common and deadly circumstances will not avoid such circumstances, and will fall prey to them more often than those who avoid them. Over time, those individuals whose genes confer on them a fear of deadly circumstances will overwhelm those whose genes do not confer such fear. There's nothing magical here; it's just evolution.
Organisms also evolved to be empathic - that is, to feel for other organisms as if the experiences of others were their own. It is often most vividly experienced between parents and off-spring, but also exists to lesser degrees in humans towards virtually any other life-form that exhibits anything remotely likely "human" behaviour. Empathy again can be viewed as providing evolutionary advantage, especially in herd/social animals like humans, in two ways. First, empathy drives us to care for the social group, which increases the odds of survival and reproduction of all members. Second, the safety of the group helps lower the stress of the individual. By lowering stress, all the group members increase the odds of survival and reproduction.
These stresses are innate and deeply visceral, extending back probably hundreds of millions of years; there's precious little an individual can do against them. But one can act to remove the triggers that cause those stresses - like working towards situations where murder is not common.
So organisms started to act so as to lower those stresses. Over the aeons, various actions that lowered the causes of those stressors gave subtle advantages to those organisms that took those actions and let them slowly overwhelm other organisms. These actions started to form sets of behaviours that distinguished one group from another.
Of course, there's substantial variability in those norms among individuals in a group. There will always be variability. With respect to the example of murder, different people will have slightly different norms about how "bad" it is. What effects that variability has on matters is something best left to future posts.
Humans, and a few other species, also learned to reason - that is, there evolved in them abilities to think temporally and abstractly, to plan, to induct and deduct consequences of actions without necessarily actually executing them, and to generalize from specific cases. Most importantly, they evolved the ability to reflect - to think about those mental models that include their selves. Humans seem particularly good at reasoning compared to most other species. This ability to reason led to the development of abstract concepts, like "murder," "good," and "evil," and so on, which then opened the possibility of developing principles of behaviour that encapsulated all the unconscious, instinctive, and evolved abilities that came from our more primitive predecessors. Different belief systems of these abstract concepts will introduce further variability in the behavioural norms, but it seems to always be variation on a theme.
Here enters "socialization." As norms within a social group are developed, new generations of individuals are indoctrinated into those norms. They spend their formative years surrounded by them, informed by them, rewarded and punished in accordance with them. As they mature, those norms become internalized, and provide a context for assessing what is "good" and "bad." By adopting and internalizing the norms of one's social group, one helps ensure one's "security," which lowers one's stress, which forms a positive feedback loop with the underlying evolved instincts at work. These norms reinforce and often tweak the innate and evolved instincts that we all have. Nowhere is the impact of socialization more obvious than in the clashes that occur between different cultures (where a "culture" could well be defined as a group with a uniform set of social norms.) Socialization will add yet more variability to the behavioural norms.
Still, some norms are so basic, so rooted in our evolved instincts, that they are universally adopted.
Thus, "murder is bad" remains a justified and reasonable generalization that accounts for most cases of "murder," in most cultures. If there are exceptions (I don't know of any), then they must be explained for this theory of mine to make sense. But that too goes beyond the draft I want to present here.
This view also impacts on the notion of "moral absolutism." The absolutist position is that some actions are always right or wrong, regardless of circumstance. The question that arises, though, is what exactly is meant by absolutism. Let's say, for the sake of argument, that the view that I describe above is correct. In that case, one might limit "absolutism" to the extent that it's domain is bounded by the limits of our evolution and socialization. So insofar as we all share a common evolution, we can safely say that, at least as far as "life on earth" is concerned, there is such a thing as moral absolutism, and that it is grounded in the mechanisms of evolution and the specific instincts that we have.
But that's not really absolutism, is it?
Indeed, I think there's no such thing as true absolutism. I cannot think of a single thing, phenomenon, or entity in the known universe that, on sufficiently close study, is absolute. One might suggest concepts like absolute zero, but even that is bounded by the nature of our universe. And as it's not clear that our universe is the only one there is, then we cannot really say (yet) whether absolute zero is anything more than a measure of a characteristic relative to our universe.
Whether absolutism, in morality or anything else, really exists, however, is rather beside the point here. More important is why we bother with morality to begin with. Why all the fuss about it? Why do so many people get so upset when "immoral acts" are committed? Why have there been so many books written about it? Why have so many individuals suffered as a result of acting in an "immoral" way?
Again, it comes back to safety and stress. The only reason I can see why anyone cares about morality at all is that a moral group is a group that is safer for its members, which lowers stress and increases the odds of continued existence. Morality is the means by which we put into thought and into words the instincts that have evolved in us, and that we cannot possibly avoid.
And it doesn't just apply to murder. As far as I can tell, there is no act carried out by a human that doesn't conform in some way to an instinctive drive mediated/tweaked/altered by socialization.