What does that mean?

One could see it as an extension of natural selection. I agree with you it's odd- but metaethics is even more slippery than ethics, so I am reluctant to pass judgments instead of making descriptions.

What happens if you replace "carnivores" and "animals" in your last phrase with "criminals" and "victims"?

If I recall correctly, he had significant affection for the likes of Genghis Khan. I don't think he would sympathize very much with the petty robber but considers the mighty and powerful as operating on a different level from normal people, with correspondingly different morality. But it's been a while since I've read his work along those lines, and so I'm not entirely confident about that. I do recall a passage where Zarathustra claimed his favorite animals were the eagle and the snake, and he approves of predation in general, I suspect.