I don't think this blanket statement is true. eg: people often live long past the point that they enjoy being alive.
> Now, eating an animal that died of natural or uncontrollable causes, or using for-real excess milk or eggs?
What's "natural" in your opinion? Do you mean old-age, disease, natural predators or something else? Humans are natural predators. Death from old age/disease is often caused by environmental factors or stress.
Calling any of this "evil" is an interesting choice. Maybe this is all just how the world works and we have yet to accept that as a species. I do think our role in the food chain needs to be sustainably defined and understood.
Next, I agree that "natural" could mean a lot of things. In this case, understanding that we're discussing moral involvement, I'm speaking about whether or not you have moral involvement in causing the death.
Paying for a burger from McDonald's is indirectly asking for the death of the cow. They wouldn't kill it if no one bought burgers, but we all know that people do buy burgers, so the blame is diffuse. We all share. But you, in eating it, still profit by the being's death, and it was ultimately a deed done for your benefit when you chose to pay them. Of course it is more direct if you pay a butcher to slaughter a cow, and far more direct if you wield the knife. Hit a deer with your car by mistake? Maybe that's like manslaughter levels of culpability.
As a sidebar, is a cow killed by a wolf a natural death? It happens 'in nature', and the wolf couldn't live without killing other things, so it is natural for a wolf to kill a cow. But... is the death a natural death? I do not feel it is. I cannot blame the wolf (as it does not know better and does not possess other actual options, like we do), but I do not think the cow exists for the wolf. To paraphrase Kant I think, each being is an end unto itself. The cow's most natural death is one of old age, i.e. what we would legally call 'natural causes'. Suppose that health care could have extended its life, well sure, but there's a limit of practicality.
Last, it would be possible to justify literally any act from the position of "maybe this is just how the world works and we have to accept it" -- so, again, sure. But slavery was 'just how the world worked' for a long time, and I feel I am better off, and I can think of a hell of a lot of other humans who are better off, for us eventually having not just accepted it. There are similarities.
Veganism is really the only choice that's morally consistent, intellectually honest, and doesn't rely on special permission. It is ultimately not really arguable that it is a terrible thing for us to do to breed them, to keep them, to manipulate them, to harvest their products, and to kill them, especially in the ways we do. One must rely on ideas like 'might is right', 'we are inherently special for reasons that must be taken on faith', or 'nothing really matters anyway, everything is relative', which are exactly the arguments that have been used to justify all past atrocities of our doing.
likewise :)
In terms of practicality, a wolf needs to eat too. Without wolves (or predators) we find over-population which leads to food scarcity which leads to suffering (queue yoda reference.)
I think there is a big difference between slavery (a dark human endeavor) and the natural order of the world. Humans didn't invent predators. At best, we conquered them and at worst, we became them while growing to become the second most dominant (and largely unchecked) biomass on the planet.
Given that plants have been shown to have feelings, I'm not sure where our idea that eating plants is morally superior to eating animals comes from. Some plants poison animals in order to fertilize their soil. Some people poison plants in order to have a somewhat pleasant looking front-yard. Many plants have developed obvious defenses against being eaten, much like animals. By eating fruit that has dropped to the ground, you are denying the plant its intent to reproduce.
I think that most humans are more capable of empathizing with other animals than with plants and that's the root of the sense of morality here. The hard truth is that no living thing wants to be eaten to death but every living thing needs to eat. The circle of life is a real thing, the carbon cycle relies on plants recycling decayed organic matter including animals.
Death is a form of suffering. Whether the creature you kill knows it lives only to die for you, whether it can see and understand some aspect of its cage or not, you took it before its time. It is all the same wrong in the end.
Now, eating an animal that died of natural or uncontrollable causes, or using for-real excess milk or eggs? Sure, fine. But those are not at all sustainable activities at a population level. We have to do things to animals that cause real harm to ever get enough of those products to sell them, even if we treat the animals way better overall. (And it's still better to push for better treatment than to do and feel nothing!)
My own mental backflip: I do still eat meat, because I'm a selfish and horrible person. I accept my complicity in the evil. But I also know my own choices will not meaningfully change demand patterns, so I allow myself the pleasure of the flavours I grew up eating, and try not to dwell on it too much. Even the Buddha was said to eat meat if it was what was served -- but of course, would request not to be served it!
Regardless, I know full veganism is the only actually ethically justifiable choice, and I believe and hope it will one day be the only realistic or acceptable choice for all of us. I will advocate for it (I hope) strongly whenever I can. No being deserves any part of what we do to them for food, and most people have pretty awful reasons to continue to eat meat; often literally at the level of, "animals can't suffer!", or, "humans are a higher being!", which is a mind-blowingly horrific thing to understand people will assert even after being given the evidence. I find the dismissive attitude appalling and try to get friends to at least see it in its real light.
Personally, once I catch wind that the tide toward veganism is beginning to turn, or that we somehow achieved affordable ethically lab-grown cheap muscle+fat tissue meats, I'll gladly turn that page. Or, once the horror again resurges strongly enough to overcome the value of shepherd's pie, thanksgiving ham, or a hamburger.
It's not much of an intellectual position, but it's all I've got. I ask that you at least be honest with yourself: the beings you eat have suffered for your pleasure.