There are few moral convictions that enjoy the same intuitive plausibility and level of acceptance both within and across nations, cultures, and traditions as the conviction that, normally, it is morally wrong to kill people. Attempts to provide a philosophical explanation of why that is so broadly fall into three groups: Consequentialists argue that killing is morally wrong, when it is wrong, because of the harm it inflicts on society in general, or the victim in particular, whereas personhood and human dignity accounts see the wrongness of killing people in its typically involving a failure to show due respect for the victim and his or her intrinsic moral worth. I argue that none of these attempts to explain the wrongness of killing is successful. Consequentialism generates too many moral reasons to kill, cannot account for deeply felt and widely shared intuitions about the comparative wrongness of killing, and gives the wrong kind of explanation of the wrongness of killing. Personhood and human dignity accounts each draw a line that is arbitrary and entirely unremarkable in terms of empirical reality, and hence ill-suited to carry the moral weight of the difference in moral status between the individuals below and above it. Paying close attention to the different ways in which existing accounts fail to convince, I identify a number of conditions that any plausible account of the wrongness of killing must meet. I then go on to propose an account that does. iv I suggest that the reason that typically makes killing normal human adults wrong equally applies to atypical human beings and a wide range of non-human animals, and hence challenge the idea that killing a non-human animal is normally easier to justify than killing a human being. This idea has persisted in Western philosophy from Aristotle to the present, and even progressive moral thinkers and animal advocates such as Peter Singer and Tom Regan are committed to it. I conclude by discussing some important practical implications of my account.