In the scientific arena, animals generally aren't even associated with ethics. I'm not sure I agree, but science is supposed to be objective. (Some biologists won't even give their subjects names for that reason.)
In the philosophical arena, we might think of ethics as something that embraces at least some animals other than humans.
In this spirit, the wolf and man both eat sheep in order to survive. Merely eating sheep doesn't make either one unethical.
However, you point out that the wolf preys on the slow and weak, thus weeding out individuals that need to be weeded out. The man does the opposite. This reminds me of trophy hunters who only shoot animals with huge tusks or horns. This may result in the evolution of animals with smaller tusks or horns - hardly a good thing.
Whether or not this is an unethical practice is largely a matter of opinion. It's obviously bad for the species and therefore the environment, which makes it unethical in my eyes. But should we make an exception for an ignorant trophy hunter who isn't aware of his impact on the gene pool?
P.S. It would be interesting to know how the philosophical community in general regards non-human animals in terms of ethics. There was one famous philosopher who believed that animals had no souls; if you kicked a dog and it howled in pain, it was just an automatic reaction, like a piece of wood breaking if you bend it. But I would imagine some philosophers have imagined animals to have souls, which might imply that they also have ethics. Moreover, some animals will ostracize members of their groups if they do the wrong (e.g. "bad") thing.
Second, humans are animals. Which begs the question, if we went back in time to a point before our ancestors had learned to make clothing or tools or build fire, would they still be "superior" to animals and therefore ethical? Or would we have to go further back in time to a point where the human brain was too small to really think ethical matters?