I honestly don't get why it's such a difficult thing to say that sapient-eating is clearly an additional evil act when paired with other evil acts, despite having some (limited) contexts where it is not an evil act if paired with other actions.
Consider, for example, that essentially all cultures (and almost surely gnolls too) will support the notion that assaulting someone is bad, and doing so as part of trying to steal something is clearly wrong/evil. Yet the very same assault--indeed, literally the exact same act or acts--can be made completely unobjectionable if certain events occur first, even in our Western society. We call it "boxing" or "MMA fighting" or the like, situations with rules and structure, with a culture of sportsmanship rather than the mere wild abandon of spontaneous violence or the cold execution of a criminal assault.
Context has always mattered, even for moral realism. Moral realism is not an excuse to turn your brain off and ignore salient details.
Hence: I see zero problem with saying "it is not evil (but remains highly unwise) to eat sapient bodies in some circumstances, but the extremely likely circumstances--committing murder in order to eat the victim, or following up murder with opportunistic consumption of the body--does constitute an independent but connected evil, one enabled but not caused by the foregoing act." Consider taking valuables from the effects of someone whom you have murdered, yet whom you know to have no heirs and no beneficiaries: there is no person left to be harmed (because you killed the only owner the objects could possibly have), yet it is quite clearly understood that this is still, meaningfully, stealing. Even if it weren't murder but instead an accidental killing or self-defense or war or whatever other "justified killing" context you like, stealing from the dead, even if you know there are no inherited to be harmed by such theft, has a wicked edge to it. And if you just so happen to have circumstances like this crop up regularly...well, once is a fluke, twice is coincidence, three times is a pattern, as the saying goes.
Also, as for the harm in consuming the flesh of sapient beings, two things. One, viewing other sapient beings as mere objects is an extremely dangerous precedent, especially given the many IRL evils such thinking has justified (like chattel slavery). Such things are the root cause of a great many evil deeds, and I have no qualms labeling as "evil" any behavior which strongly encourages such views. "When you start treating people as things..." as a certain Sir Terry wrote.
However, some find such broad principles insufficient as an example of evil, despite their serious issues. So, two: the relatives of the deceased. Just as stealing from a dead man harms any inheritors he may have, damaging the body of a dead person harms their friends and loved ones. Funerals, after all, are rarely for the deceased, but instead for their survivors. If a sapient's corpse is "just an object," then in a very meaningful sense that object both belongs, and is supremely important, to those who cared about the person the corpse was. (And this, we can note, is a major difference between the death of sapient and nonsapient creatures; even a very intelligent animal, to the best of my knowledge, at most follows routines associated with the dead person, as in famous cases of dogs going to a master's grave or "going to meet" their deceased master every day at the train station or the like, but quite often the animal simply forgets over time. Humans do not merely grieve, we demand closure, desperately rail against the possibility of death for a loved one, conduct elaborate search efforts until they clearly become futile, and often never truly recover if denied conclusive proof of death. While it may not be the case that all sapient species do this, I must say I find it rather unlikely that any species that did not do this would form stable social groups as humans have, and thus develop an overall culture as we would understand it. (Familial packs/troupes only, no "tribes," and certainly nothing like common language across disparate, geographically isolated populations with no meaningful familial connections.)
