If a thing cannot suffer or flourish and it has no interests to consider then how you treat it raises no moral issues in itself. But if creatures do have interests then how you choose to treat them does raise moral issues. We can ask whether it would it be right or good to increase suffering or to promote flourishing. In general the answer seems to be “it depends.” What it depends on seems a thorny question, and one influenced by cultural and personal attitudes. There are a couple of trends maybe:
One is enlightened self-interest. If we benefit from looking after each other then, other things being equal, it seems to make sense to do so. This might encourage the establishment of some principle of reciprocity. This may not necessarily lead to universal principles. We may see benefit in some areas and not others which may limit or constrain our concern. This is a kind of business model of morality. It lacks some charm, but I suggest that this is often how it is from the national to the personal level. From trade deals (or wars) to gang codes to personal relationships, there is often a cost-benefit analysis; what am I putting in, and what am I getting out? Am I being cheated or exploited? (I do this, this and this, what do you do)? For example, much of the concern about immigrants reflects worries that they will take more than they give. I have heard arguments that suggest that they do, in fact, usually give a great deal. Again, when a horse has reached the end of its working life then, perhaps, I can eat it. And it is reflected in discussion about whether animals are better off under our control. Debate from the perspective of self-interest tends to favour those who have power, (e.g. political, economic, personal) and who see no need for or benefit from compromise (might is right).
A second trend is emotional. We empathise and we care. H_k_s describes a good relationship of mutual self-interest with his/her cat. But I suspect the cat has come to be loved and would be cared for even when old and deaf or unable to hunt. (I may be wrong, I really don’t know for sure). But to the extent that we empathise with, and care for others, then we will correspondingly be concerned with their welfare. I think that by and large this is easier with some animals than others (e.g. it is, perhaps, easier with cats, dogs, horses etc.) which experience indicates may have subjective experiences not too different from our own. It’s harder (not, perhaps, impossible) with fish or spiders (perhaps it’s me) and harder still with viruses. Your point with regard to Nagel is well taken. I think we are forced to rely heavily on our intuition, and this may not be reliable. (What else can we do)? When we choose whose interests to protect first it will often come down to those we care about most.
It may be that ultimately, then, moral decisions are in some ways selfish. They hinge on perceived physical and emotional consequences that we experience following our treatment of others. This reflects cultural and personal influences, but also human ones, rooted in our human capacity for empathy and caring, and when we see this capacity reflected in other animals it ignites a spark of recognition. Ultimately what we deem moral reflects subjective preferences. Telling people they should promote the welfare of those they feel no concern for, and see no benefit from troubling with, will likely be an uphill struggle. I think it may help us to decide how to treat with each other and with others if we understand ourselves, and others, more clearly. And if we are looking to create a foundation for a moral philosophy, one that people will relate to and act upon, then that understanding is a reasonable place to begin.