It seems to me that you are confusing utility with morality. The fact that eating human flesh might not be good for us (which, being a questionable claim, let's

It seems to me that you are confusing utility with morality. The fact that eating human flesh might not be good for us (which, being a questionable claim, let's take for granted for the sake of this discussion) is beside the point. The question is not "is it good for us in a dietary perspective?" but "even if it were good, would it be acceptable?". A consequence of what you are saying is that it is impossible to claim a higher ground against a murderer if the murderer doesn't benefit from the murder or if an accuser can't make a case for it. Besides, benefits are relative most of the time: you say there isn't one, but someone else may just as well argue there is. But if you absolutely need a benefit, so be it: imagine someone eating human flesh simply to satiate one's appetite for it. It would still be a morally questionable practice because of other values put into question. Anyway, this was a thread about fish feeling pain, so I insist: maybe there are many species we should treat in a different way, independently of whether they feel pain or not. Some scientists have reclaimed the statute of non-human personhood for great apes and cetaceans, for example. So killing them painlessly still raises questions.