In the wake of the Newtown, CT school shooting, philosopher Russell Blackford asked why news reports emphasized the number of children killed, rather than the total number of victims, as if adult lives were worth less than children's lives.
This question really probes our ethical and emphatic priorities. On one hand, few people would disagree that lives of adults are intrinsically as precious as lives of children. Even those who think in terms of years of potential life lost could not conclude there is much difference, on that basis, between a loss of a child and a young adult such as a teacher in her 20s. On the other hand, most people, including me, have a markedly stronger visceral reaction to a murder of a child than to a similarly executed murder of an adult. My moral intuition tells me that the murderer of a child is more culpable. Why?
One could try rationalizing the difference in terms of the loss suffered by the survivors. For most people, it is more painful to lose a child than to a parent or a sibling. That is probably an adaptation to our mortal nature: everyone dies, and the best we can hope for is that deaths will follow the sequence of generations. However, this explanation does not work for several reasons. First, although adults are generally well-adapted to accept the death of their aging parents, a parent's early death can still be a terrible loss for a child of a certain age (old enough to understand death, but too young to be emotionally detached from the parent), so a strict hierarchy of grief is far from clear. Second, hearing of a death of a child is not any less heart-rending if we also learn that the child was an orphan whom nobody loved and thus nobody is grieving. If anything, that additional information may sadden us more. And third, an introspection into my reactions convinces me that a strong revulsion at the act of killing a child comes before, and is independent of, any thoughts of how horrible it must be for their parents. I am quite sure my moral calculus in this case is driven by outrage (anger) rather than empathy, and I suspect this is common, perhaps nearly universal.
My theory is that our moral calculus of culpability is driven by fundamental notions of fairness, that it is evolutionarily conditioned, and that it is ill-adapted to the modern world with a fundamentally changed technology of killing.
Before we had guns, or even bows and arrows, killing another person generally required overpowering them in close contact. For millions of years, it meant using bare hands (and feet, and teeth), then maybe sticks or stones, and in the last few tens of thousands of years also axes and spears. But, basically, to kill someone, you had to fight them and win the fight. You had to be stronger than them.
Or you could catch them asleep and kill them when they couldn't defend themselves. But that's unfair, by a universal human (and broader) standard of fairness, and thus universally condemned. While killing in a fight may feel more or less justified, killing someone in their sleep carries a strong presumption of a cowardly murder. Fairness requires that the victim has a chance at defense, and that, in turn, requires a reasonable balance of strength. I believe this describes pretty well the moral sentiments of primitive humans regarding violence within their community (tribe), but outside of one's immediate family (where some different rules may apply).
Now, men are naturally physically stronger than women, and adults are both much stronger and smarter (more experienced and cunning) than children, which means that a man attacking a woman, or an adult attacking a child, is a fundamentally unfair act, somewhat akin to a man attacking a sleeping man. Thus the human sense of fairness ranks an attack on a woman as more wicked (or at least more cowardly) than an attack on a man, and an attack on a child even worse.
It would follow that killing a child is worse than killing a woman, which is worse than killing an unarmed man, which is worse than killing a man with an axe. And this is consistent with the prevailing moral sentiments of today: it is usually considered OK to kill an enemy soldier, but not OK to kill a civilian, even worse to kill a civilian woman, and worst of all to kill a child.
(A reverse calculus may apply to perpetrators. For example, Judith kills Holofernes while he is defenseless, in a drunken stupor, escapes alive, and is celebrated as a heroine. Could a man get away with it? Male mythical heroes who kill by deception usually don't kill a sleeping enemy, and often have to pay with their lives.)
Modern weapons (and poisons), however, make this moral calculus dubious. Bodily strength has little to do with the capability of modern humans to kill, or to defend themselves. A small woman with a gun can easily kill the strongest man. Or she can lace his drink with cyanide. Either way, his muscles provide no defense and he is objectively in no better position to avoid death than a child. Therefore, in any situation involving an armed killer and an unarmed (or armed, but unprepared) victim, the victim's strength, or sex and age on which the strength largely depends, is irrelevant for the fairness of the situation, and hence, by the logic of fairness established earlier, for the culpability of the killer.
But, as is often the case, our moral sentiments have difficulty adapting to the world dominated by technology.