Animals inhabit a liminal space, where their moral status is unclear. As sociologist Clinton Sanders says in his book Killing with Kindness, "[The] relative lack of discomfort surrounding the death of animals in western culture derives largely from their traditional definition as objects." In other words, they are not "persons" in the morally relevant sense. They are simply pieces of property. "Nonhuman animals are culturally defined as a generic group and, as such, relegated to the social category of 'nonpersons.' Companion animals, however, exist in the liminal space between object and individual being." Those who live closely with animals often come to know them and think of them as "persons" rather than objects. Thus orchestrating their death takes on additional moral weight and becomes morally fraught. Sanders continues, "the purposive killing of companion animals is a matter of ambivalence, though this ambivalence typically has a considerably different character than that which accompanies the euthanasia of terminally ill, permanently unconscious, or severely damaged humans." The latter also reside in a liminal realm between life and death, but their status as potential humans or once-human gives them a heavier moral weight than not-human, never-human animals.
The killing of nonhuman animals carries considerably less moral weight than killing humans—this is obvious. But some animals are more difficult to kill (morally speaking) than others. Why is this?