it's okay to eat animals because "it's survival of the fittest", i.e. what evolution prescribes
This is saying that we are the fittest, because we can eat animals, and we therefore should eat animals because we are the fittest. We can do it, so we should do it because that's how it has been done.
Evolution is a description of what occurs when you combine reproduction, selection, and mutation. The whole point of the theory is to demonstrate that some sort of overarching design or purpose is not needed to explain how animals end up so well adapted to their environments. So to then say that this is in some way how we can infer some "design" or "purpose" is exactly wrong. Evolution is not a prescriptive framework.