Just because something is natural it does not mean it is ethical and that we should still do it. We cannot look to nature for moral advice as nature can be wrong and twisted in our human perception of what is moral.
Even if one were to accept that all that is natural is therefore ethical one can argue that;
It is not natural as we are primates. Of the 633 species of primates humans are the only ones who eat meat (chimpanzees too on occasion). Eating meat was an evolutionary fault rather than a natural process of evolution.
Furthermore, while other animals eat each other as they need to for survival, humans eat animals not out of necessity but merely because they want to. That is wrong