One view that is held by some of those who think about animal ethics is that it is not intrinsically wrong or bad to kill a non-human animal. This is a view supported by many utilitarian thinkers, such as Peter Singer. What makes killing a person wrong is that they have plans for their lives, desires about their future, and a desire to go on living, and all these things are thwarted by imposing death on the person. Animals, on the other hand, are said to live in an "eternal present," they have no concept of the future, and perhaps are not strongly connected to their future selves, such that ending their life is not much of a harm to them. On this view, it is still wrong to unnecessarily harm them, because it's worse for them to have bad experiences than positive or neutral experiences, but we don't have any obligation to refrain from killing them.
It's important to note that even if you hold this view I think there are still decisive reasons for being vegan, as I argued in a recent post. But I think this is an extremely bizarre position to hold, with implausible implications and a very weak intuitive basis. I think people only find it plausible because of an exaggerated view of the metaphysical differences between humans and non-humans, and because it helps to mollify the scale of the wrongs done to animals.
To see why this view is mistaken, first consider the idea that the fact that we have plans for our future is a significant factor in the wrongness of killing. While this might sound plausible at first, it would seem to imply that it's more wrong to kill someone with lots of plans for their future than someone with a few. But most people tend to think that killing someone who is depressed about their life, and doesn't see much of a future for themselves, is just as wrong (all else equal) as killing an ambitious person with many future goals. A severely depressed person may even want to kill themselves, but the fact that they have decided not to have a future does not give us the right to kill them, or even significantly mitigate the wrong of killing them. I think upon reflection the connection between plans for the future and the wrongness of killing is tenuous at best. It might be a component of the wrongness of killing, but not an essential one, and certainly not the difference between a permissible killing and an impermissible killing.
Gary Francione has made similar points, and he draws on the case of individuals who have transient global amnesia, and can only conceive of themselves in the present. Obviously, if a human has these qualities, we don't think it becomes permissible to kill them. But perhaps it's hard to imagine this case, and our intuitions might not be trustworthy on it because such a condition is relatively alien to us. It's possible we might be mistaken about the wrongness of killing of someone with tranisent global amnesia (I find this a much less plausible concession in the case of the depressed person in my example, because depression is much easier to imagine.)
But what could it mean that we could kill an individual without a sense of the past or future, and not wrong them in doing so? We must assume that the life they would continue to leave would be valuable and worth-living, because we're not discussing cases of euthanasia. So why would it not be wrong to deprive them of their valuable future?
It seems to me the only way to make sense of this view is to believe that, due to the individual's lack of a sense of past and future, there's a meaningful sense in which the valuable future will belong to someone else, someone who does not exist yet. Almost everyone agrees that we don't have an obligation to bring new individuals into existence; that is, there are no obligations to make non-existent persons into existent persons. How could a non-existent person have anything but non-existent rights? (There are some theoretical problems surrounding this view, though most believe that this premise is relatively certain. I would take it to be a strong objection to a view if it implied that non-existent persons did have rights, especially the right to come into existence.)
So we can see how this explanation makes sense of the "kill animals isn't wrong" view. If all animals live in an eternal present, each new moment is in some sense a new animal, and so killing (painlessly) the animal doesn't wrong it, it just (permissibly) prevents a different animal from coming into existence. An animal, on this view, is not a wholly unified individual, but rather a series of connected, but morally unrelated experiences. But now, I hope, this should strike us as a rather implausible view. It would suggest that the dog I take a walk today is not (in a significant sense) the same dog I took on a walk yesterday. It just inhabits a similar form and has a similar psychological make-up. On this view, the dog is in a certain sense, not a dog, but a succession of experiences of dog-ness that are generally cohesively organized.
I find such a view very difficult to accept. Of course the dog I walk today is the same dog I walk tomorrow, and the same dog we rescued from the shelter. The view I've described requires a serious shift in our sense of what animals are, and I think gives us little compelling reason to do so. Perhaps the psychological connections that constitute identity over time are weaker for some non-human animals than for humans, but they also vary across the different forms of human life, and in these cases the differences seem to have little implication for our judgments about the wrongness of killing. And the only reason we have for adjusting such views is that people want to justify the killing of animals.
Like many of my arguments concerning animal rights, I find this one to be rather straightforward. We should not kill animals because they have valuable lives which are wholly their own that they have an interest in pursuing. Is it better for an animal to have one happy year of life or two? The answer is obvious. What's puzzling is why it isn't obvious that killing the animal, and thus depriving it of many more valuable months or years of life, is seriously wrong.