Every time I feed crickets to Froggy (my son's pet frog), I think about this. Let me clarify that I am not sentimental about animals in most respects. I don't like to see them hurt, but I like to eat meat and I see nothing wrong with it. When it comes to crickets, I dislike them strongly. I even have some stories about why I dislike them so much. Still, when I open the container and drop a few in with Froggy, I can't help feeling like an executioner. They seem so happy to get out, they hop around, and then Froggy pounces and devours one whole. Tonight, he happened to have a large one that didn't all go in his mouth at once, so the cricket's head was sticking out, still alive, as the rest of it was being eaten.
I don't have a lesson out of this, except to say that humans are well-behaved by comparison. Some people, especially animal-rights activists, like to point out how close humans are to animals: we share 98% (or is it 99%?) of our DNA with chimpanzees, much of our behaviour is similar to social animals, animals can use language and make tools like humans, and so forth. Ironically, however, they then judge human behaviour by totally different standards: if any person hurts any animal, it is an outrage against — er, against humanity. Humans kill and eat, they kill systematically and en masse, they are crowding out other species; but it's all part of life. Animals kill and eat. Ants raise other insects as "cattle," and, while I haven't been able to find a case where they eat their herd (they use them to produce "milk"), they also collect slaves. And evolution works precisely because some species get crowded out or out-competed for resources. I just wish these people would be consistent and judge animals by the same standards that they use to judge humans. If hurting an animal is bad, then many animals are bad. Animals don't have the capacity for empathy? It's hardcoded into their genes? Then humans are indeed different, and we can't apply the same standards, ergo a rat, a dog, and a pig are not the same as a boy. They may all feel pain, but only the boy is capable of reflecting on his pain and thinking about how it relates to pain in others.
My personal view is somewhere in the middle. I think humans are very close to animals in behaviour, and that's not likely to change unless our DNA changes. I also think humans are different than animals because of our capacity for abstract thought. I think this means we should be nice to animals; we should protect endangered species and we should strive to avoid causing pain to animals. We should protect animals precisely because we are better than them, because we are capable of being magnanimous. I view gratuitous destruction of animals (e.g., dogfighting) as perverse, but accidental destruction (overfishing, habitat destruction) as an unfortunate result that we should try to avoid — not a sign of some essential human depravity. I also believe that some animals are more worth protecting than others: mammals more than fish, fish more than insects. If you can work up the same sympathy for a dying fly that you can for a dying dog, you clearly have a different set of sensibilities than I do. (Animal rights activists recognize this, at least implicitly, insofar as they make a greater effort to save mammals such as dolphins and polar bears than frogs or fish. Explicitly, however, it is not clear on what basis they could argue for drawing such a distinction, given their premise that animals and humans are fundamentally the same.)