Socio-evolutionary psychology gives us some great insight as to why humans tend to be not okay with killing other humans, but are okay with killing and eating animals.
Humans, being social creatures, perceive their environment in a way as to simplify it (because we want to minimize cognitive processing); this leads us to establish guidelines or rules of thumb (called "heuristics") that generally work for any given situation, but sometimes lead to cognitive biases. Social psychologists tend to think of people as "cognitive misers," trying to optimally use cognitive load.
One such way to conserve cognitive capacity is to form associations of the self with other social creatures, and to put other creatures (human or otherwise) into various other groups. This is a key point to the explanation of phenomena such as racism, sexism, ethnocentrism, nationalism, and any number of other -isms that we see in society. As a mental exercise, write "I am:" on a sheet of paper, and then start classifying yourself. Here I go:
I am:
-- young man
-- Christian
-- blue-eyed
-- American
-- musician
-- programmer
-- social psychologist
-- x86 and vL
It's really neat the different ways you can put yourself into various groups.
Now, when we classify other people, we have an incomplete picture. Furthermore, because we know many people, we tend to want to simplify the process of classifying them, because we have a limited amount of information storage and processing time. So, I might classify iago as:
iago is:
-- Canadian
-- programmer
-- security person
-- x86 and vL
-- smart
I don't know much more about iago. However, I can already say that iago and I match up on at least two social groupings. I try not to toot my own horn frequently, so I'll forego claiming that I'm smart. However, during the course of this exercise, I can at least assert that this quality is one I would like to have, if I don't already. Socially, then, I tend to get along with iago better than some others, because I can identify that he is in several of the same social groupings that I've constructed (I'll call this cognitive mapping and self-identity).
Now, I don't know Rule very well. I know that he's in vL. But I also know that I generally have perceived his posts as arrogant. (That's not to say that mine with him haven't been either. But thanks to the "false attribution error," I've assigned the "arrogant" characteristic as due more to his personality than the circumstances surrounding the perception). So I will say:
Rule is:
-- arrogant
-- vL
-- mathematically-adept.
Rule's in vL, which kind of puts him in my ingroup, but since vL is pretty low on my "me" list, it doesn't really do much for my perception of Rule. When you add in the "arrogant" perception, which I have a bias against, Rule tends to be in the outgroup.
iago is clearly more a part of my "ingroup" than Rule, and so I would therefore give iago preferential treatment (an effect of the ingroup bias). Additionally, because Rule and other people who appear to argue along with Rule's positions, will become more homogenized in argument type, style, and content, which leads to the "outgroup homogeneity bias." Have you ever heard, "All those Chinese people look the same to me"? That's a killer example of the outgroup homogeneity bias.
It's one of the reasons that you can't easily remember the faces of one panda and distinguish it from another. It's also one of the reasons people tend to have difficulty remembering faces of people they barely know - they haven't had the opportunity to learn much about these people.
Where do these biases come from? Evolutionary psychologists suggest that such biases exist due to hard-coding of evolutionarily-advantageous rules. It is beneficial, for instance, for people not to kill each other, because as a species, humans can only procreate and reproduce their genetic material with other humans. Furthermore, species see each other as competition for limited resources. Why do people tend to see other people as less competition than animals? Because people tend to see each other as within an ingroup that is ranked more importantly than an ingroup with unknown animals (pets not included).
Developmental psychologists may offer an alternative explanation, partly due to imprinting: when a new animal is born (referring to non-humans, now), many "imprint" the sight of their mothers, and they know that these creatures are permittable and desireable to be with.
Animals (food ones) are in a much more significant outgroup than other humans in that animals, particularly domestic ones, cannot care for themselves, and, well, they can't be reproduced with (though some people try).
It's with this outgroup bias that people tend to justify doing such things to them (animals), and it most likely stems from an evolutionary behavior coded into us.
Such justification doesn't necessarily make the viewpoint right or wrong. I'm just being thorough.
