The Predation Defense
Lately I've been torturing my family with the question: what would the world be like if all animals were herbivores? I'm pretty sure the answer is: not as good. Evolving to hunt makes a species develop all sorts of perceptual and cognitive strengths, and likewise, evolving as another species' prey. I can't see how a world filled with herbivores, and only herbivores, could have minds as sophisticated and varied. Predatory mammals would be missing, but also carnivorous birds and fish. You could wish the world had an intelligent and benevolent creator that would just make clever, diverse, bounteous animals, so the misery of prey being eaten could be avoided, but that's not what our world is like. In our world as it is, predation is a necessary phenomenon that's shaped species for the better. There aren't just vastly more species, because of predation, and more "filled" ecological niches, but the species that exist have more and better skills. Or so it seems--I'd love to have an opinion from an evolutionary biologist given to speculation!
If predation is a force for good, one thing that follows is that when we think of lions eating zebras, or raptors eating mice, or sharks eating baby whales, we should not disapprove. Few will outright morally condemn a lion but sometimes the reason for avoiding condemnation is not the right one. It's not just that the lion isn't a moral agent, so can't be condemned. A lion is not like a toddler drowning his baby brother--doing wrong, but not blameworthy. It's also not just that the lion, as an individual, needs meat to survive. The imperative to hunt wouldn't go away, if the lion had a nutritious vegan alternative (faux zebra?). Because (to repeat) predation is a force for good -- without it, there wouldn't be some of the traits, abilities, and species that seem most valuable. Yes, of course, predation does cause harm to the individual animal eaten, but it's still overall a force for good (in a world like ours, not run by a perfect being).
Now, what does this paean to predation mean for us? We are predators too, of course--since we are omnivores. If predation is on the whole a force for good, we should at least not be disturbed by the impulse to eat meat or drink milk. These are not the least bit like impulses to torture animals for fun, or molest children, or rape women. Your inner lion is okay, not criminal or pathological or malevolent. So much for self-esteem! We should have it, even as we feel attracted to the smell of barbecuing meat, on a hot summer night. But should we go further--should we consume meat and other animal products?
I don't think I'd be in worse moral fettle than a lion, if I were running around in the wilderness killing rabbits for my dinner. As a moral agent, though, I'd have to think through why I was doing this as well as how. I'd have a duty to kill the rabbits as kindly as possible, and to kill no more than I needed. Moral agency does, then, make a difference, but not the difference sometimes claimed: I don't have to refrain from killing rabbits, because I'm a moral agent.
But what about meat consumption in a more typical case? Predation is a force for good in nature, to the extent that it makes nature more varied, bounteous, and mentally sophisticated. It works wonders in an ecosystem. But are we humans really still part of an ecosystem? We seem more like destructive aliens, relative to every ecosystem. Predation is generally a force for good, but predation via domestication, as practiced today by our extremely populous species, is a special case. Its impact is just the opposite of classic predation--we plus five or six domesticated species now dominate the biosphere, and these species have been bred so that they've lost mental acuity. They're so numerous and dominate so much land that they're a tremendous threat to biodiversity.
When all is said and done, the predation defense for meat-eating seems to exonerate the impulse to eat meat, and even acquit actual meat-eating in some conceivable cases. But domestication throws a wrench into the works. Human, predation-via-domestication is a very different thing from predation as it functions in a healthy ecosystem.