The OPs point rests very heavily on a total moral equivalence between humans and animals. But not everyone believes so, nor do I believe they need to believe so. Humans are humans. Animals are not. We're all living creatures, sure, and we all ultimately come from the same source. But that doesn't mean we're the same. Humans aren't plants, either. Humans are creatures that are evolved to be omnivorous. If someone wants to make the choice to eat only plant matter, fine: it's a free country, if you don't want to eat meat, no one should force you to. But we are what we are. Eating meat is natural. And while I don't know about aliens, I can certainly say that when I hear of people getting eaten by large animals, I may think it's a tragic shame, but I don't think the animal was immoral for eating a person. That's the way they are, too.
Any time you eat something, you killed an organism to do it, unless you spend all your time carefully gathering nuts and fruits, and don't question too much where your other nutritional supplements come from. Plants or animals makes no difference, morally, as far as I am concerned.
I do believe in some fundamental decency to other creatures, though, so in addition to keeping kosher, I try to only eat organic, free-range/pastured meat, and even such factory farmed meat that manages to somehow be called kosher, I do not eat. That, to me, is more than enough moral duty.
I eat meat because in reasonable amounts it is nutritious, and I enjoy the taste of meat more than I do most plant matter. I see nothing wrong with that. And I can't say much for fundamentalist vegetarian or vegan proselytizing, either: if there is anything less apt to make me eat less meat than having someone shout in my face about how morally corrupt I am for eating meat, I can't think of it. It really makes me want to have a burger, is what it does.