Does my proposal as to what makes killing another human being generally a major moral wrong in any way help us with deciding what, if anything, is wrong with killing non-human animals and foetuses?
I believe it does help. It seems reasonable to believe that many animals share in common with us that they have things they want to do in and with their lives or which they may come to want (either again or for the first time). Certain of our killings of them clearly maximally unjustly prevent their realization of such life-purposes (or if this appears too grandiose a term, with the desires to do things which they experience). For instance, to kill animals which have these similarities to human beings in the course of pointless or duplicative experimentation, in the course of providing cosmetics, furs and other items readily producible without such killings, or merely for sport, is morally wrong according to the account I have proposed. Indeed to kill for food animals which it is reasonable to believe have such desires (and not merely interests) will be justifiable only where no adequate alternative food supply is available and the food is needful either immediately or for some reasonable future period if stocking up is required by the exigencies of one's situation. Where there is no reason to believe of some living being (say a mosquito or a tree) that it possesses the characteristic I have been concentrating on there will on my account be nothing intrinsically wrong in killing it. This is not to say that other instrumentalist considerations (e.g. to do with ecological effects) will not be relevant. Similarly, should anyone doubt that the animals human beings typically eat for food have life-purposes (even in a rudimentary form), this will not show that questions of morality have no relevance to our treatment of them, since other principles such as those advocated by Peter Singer in Animal Liberation (New York: New York Review, 1975) here assume relevance (e.g. ones to do with the painfulness of the methods of rearing and killing.) It is worth noticing that my proposal does not rule out killings which have the effect overall of fostering the wants of the largest subset of some group like a wild herd where otherwise the wants of an even larger subset will be thwarted. Systematic cullings in the absence of feasible alternatives, therefore, may be morally permissible.
(Robert Young, "What Is So Wrong with Killing People?" Philosophy 54 [October 1979]: 515-28, at 526-7)