Why is it wrong to kill another person? The answer "just because" comes to mind, but that, of course won't do... I love this question because it raises so many more, and its tendrils reach out into so many contemporary moral issues. It also won't do to say "It's wrong to kill because god said so" or "because life is sacred" for the same question arises: Why? Then there is the golden rule approach: we shouldn't kill others because we don't want them to kill us (I use this prudential version of the rule because on the traditional version: do unto others... I want to ask, again, Why?). This might do, but I'm not sure it captures the morality of the injunction in individual cases.

Enter the hermit, that sublime hypothetical human, orphaned from birth, who lives alone in the woods, never plans to have interaction with any other human, has no family, no friends, and is content with himself alone. You stumble across his abode while lost in the woods one day and he grudgingly allows you to stay the night. Would it be wrong for you to kill him? No one else will be harmed, no one will mourn, no children or parents or pets will suffer, he's not going to discover the cure for cancer, and no friends are going to search you out and kill you in revenge, nor will the police for that matter.

The crucial point in the question of killing which is brought out most clearly by the hermit thought experiment is this: Everyone agrees that you can build a case for the wrongness of killing around Harm, i.e., you can argue that harming an innocent person is wrong, to kill an innocent person is to harm them... The hard point here is: how does killing someone harm them? (for we can easily suppose that we kill the hermit painlessly). Once they are dead, they can't be harmed (can they?), and the hermit example removes all other external factors. So why is it wrong to kill hermits?

Don Marquis has a beautifully simple answer to why it is wrong to kill. It is wrong because it takes away what is most valuable to an individual, viz., their future. Not that this approach is without its own problems. If you take something from someone that they will never (in principle) miss, have you done them wrong?