The justification for killing a fish or a cow is that they're food which we have evolved to eat.
So are other humans.
For Papua New Guineans perhaps, but for the vast majority not so much.
Are you seeking to imply that the people of Papua New Guinea evolved separately to the rest of humanity?
Or are you just averse to explicitly admitting that your statement has been shown to be mistaken?
Clearly there must be other justifications for eating meat than our evolutionary history, which is demonstrably irrelevant.
That eating certain kinds of meat is frowned upon (whether that is human meat, or horse meat, or any other species meat) is completely unrelated to our evolved capacity to digest and even thrive upon a diet of that kind of meat.
Humans can eat the meat of any mammal (including humans), most fish, many reptiles, and most avians. This tells us exactly zip about whether eating any of these is
justified.
I have no doubt that I could, if
justified, survive by eating human, or rat, or snake; I equally have no doubt that I would need some pretty extreme justification for doing so, well beyond my mere evolved capacity to do so.