I'm not saying that they exist solely to be eaten. In all honesty I don't think there is any real reason to the existence of anything, humans and animals exist because they can not because they're needed. What I'm saying is, there is no real laws in nature that says that one species should not eat another. This is the core part of many animals survival. We're part of the food chain just as much as every other animal in existence.
If there are no "real" (or natural) laws that say one species should eat another, then there are no laws that say that a more advanced form of life can't eat us, so you shouldn't be against that either.
Appealing to natural laws to determine what humans should do wont work at all, that's why we have moral laws.
I think the world exists without purpose and without morals. No animal or man exists for a purpose, they exist because they survive. The world does not need any creature to exist, the creatures of the world need each other to exist. We survive, and many of us survive by relying on other animals. We don't need to, we want to, for our survival. Morals are a man-made concept and my morals are defined by doing what benefits humanity, while if having to hurt others for that goal, do it respectfully and never in unneccesarily sadistic ways.
Of course morals are man-made, and if you define morals like that doesn't mean that another person should do it in the same way you do. Actually, they probably won't define morals in the same way you do, but you have to accept and respect them as long as they do not interfere with your moral interpretation of the world.