I recently found myself holding the following views, which seem to be in contradiction:
It is morally wrong to kill a human being which wishes to live; in particular it is wrong to kill a baby or an elderly person or a person who wishes to live. This rule is independent (or very weakly dependent) of the intelligence/capacity for abstract thought of the human being in question.
It is morally acceptable to kill a healthy animal which wishes to live, for the purpose of consuming it as food (in particular, in a situation when not much is gained from the kill); arguably many animals killed this way experience overall more suffering than pleasure in their lifetimes.
Some animals (including some commonly consumed, say cows) exhibit conceivable signs of conscience to a larger degree than some human beings (notably, the impaired ones, in extreme conditions, suffering from serious illness, etc.): they are able to interact with their environment, have some sort of memory, experience pain, can learn and respond to the environment in a reasonable way. (I'm not including abstract thought here, and possibly some other items, because an infant does not seem to have it, nor will it ever have if we additionally assume it has some sort of heavy mental illness).
The only conclusion which I was able to reach was that 2. has to be wrong. (It also seems possible to argue that killing human beings is acceptable under certain circumstances when their thinking is impaired, but somehow I'm inclined not to go in that direction). I initially tried to think that killing animals is not immoral because of the lack of their sentience, but then it was pointed out to me that people in many cases exhibit hardly any signs of being sentient (as in the conditions explained above, or even any extreme condition where panic/instincts take over), and that some animals exhibit more such signs than men.
One could state that 3. is irrelevant (and not contradictory to 1. and 2.) if one claimed that the value of human life stems not from the fact that a person is conscious, but for example because people have souls. However, I can only imagine such an argument on religious grounds, which I would like to avoid. So far, this is the only way I see to hold 1. 2. and 3. I am quite convinced about 1. and 3., so this leads me to a believe not 2., and hence it is immoral not to be a vegetarian, among other things. This is quite a strong shift for me.
However, there is surely a lot of possible arguments I am missing here. Hence, I would be very grateful for letting me know if there is any consistent way (not relying on religion, or anything irrational) to hold all 1. 2. and 3. true. What would be a good place (book/article/webpage) to investigate the issue?