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?
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.
When it comes to death, there are different categories of animals. There are animals who are nonpersons, whose deaths really don't matter (the animals we eat and use). And there are animals who occupy some borderline status, our companion animals. But even these categories can be quite fluid. Animals of the same species could fall into different moral categories. Some dogs are treated as subjects while others dogs are treated as objects. Although social role will certainly influence moral status, this is also imprecise. Some dogs kept as pets are treated much more like an object (without moral worth) than the dogs in a well-run research lab.
Even within the category of "object" there are oddities in how we think, or don't think, about animal death. While teaching bioethics at the University of Nebraska Medical Center, I served one term on the Institutional Animal Care and Use Committee, before I and they realized the folly of this appointment. The whole experience was surreal, but especially the lunch-time meetings. I found it utterly bizarre to sit in a conference room watching members of the IACUC wolf down ham and turkey sandwiches—all catered by the U—as they talked about how to best protect the animals used by the institution for medical research. They, in turn, thought I was wacko, and highly suspicious, because I wouldn't touch the ham or turkey and had the catering service bring a vegetarian lunch.