Which is probably why it makes you sick and weak!
Here's the real rule: cooperate with cooperators, defect on defectors. This is socket rogering, not rocket surgery. Everyone can understand it.
Animals cannot cooperate. They can't read a contract, nor could they follow the contract even if they did.
This means animals are inherently defective. You're supposed to defect on them. Thus, if you want to eat one, just do.
Vegetarians say it's mandatory to cooperate with this defector, and immoral to defect on this defector. Vegetarianism is defection, and you should defect on vegetarians. The only reason you can't kill and eat a vegetarian is that cannibalism spreads disease. Vegetarians are parasites; don't make it easy for non-abstract biological parasites by eating them.
Note that technically speaking the real rule isn't really morality, because morality isn't a thing. Rather, cooperating with defectors and defecting on cooperators is analytically imprudent. Always costs more than it is worth. Always constitutes rebelling against Reality.