Most of these situations are wildly contrived to stir people up and be shocking.
1. Predator sophonts who see other sophants as just another meal.
Psychopaths. So, this happens. People like Hannibal Lecter, Jeffrey Dahmer, etc. exist, unfortunately. They're suitable as villains in plots, sure, but anyone who thinks these people are OK should turn themselves in now.
2. Sophonts who eat other sophonts situationally - winner eats loser of battle for example.
Barbaric primitive cultures. The Meso Americans' royalty ate the hearts of people who were sacrificed. No one's suggesting that this was by any means OK to do.
3. Sophont is a 'tasty' delicacy to another sophont.
How do you discover this? I found a silly Tolkien fanfic ages ago (like 25 years ago, and can't find it now) where Gandalf found hobbits addictively delicious, but it was a humorous fic not meant to be taken seriously, and even then, he felt bad about it. And I don't remember exactly how he made the discovery. I guess you could find this out in some sort of Donner Party-like tragedy, but that was a matter of survival, not a choice, and the survivors did not continue the practice.
4. Sophont is a necessary medicinal source for another sophont that must be cooked and ingested orally.
So, this and the rest are wildly contrived and if situations like this actually happened, it should work a lot more like an organ donor program than what's being described.
5. Sophont ingests another sophont because: aphrodisiac, psionic booster, physical/mental booster, illegal drug effects...
Ditto above. And what about the sophont causes this? Are there not other substances from other critters on their native planet that cause the same effect? That seems hugely unlikely. Humans, as a species, are very like most other earth species, which is why animal experiements can be usefully predictive. Coconut water can be used for human blood (
https://pubmed.ncbi.nlm.nih.gov/10674546/), how unique is anything?
6. I'm sure there are other reasons.
Psychopaths, see above.
1. Predator sophonts who see other sophants as just another meal.
2. Sophonts who eat other sophonts situationally - winner eats loser of battle for example.
3. Sophont is a 'tasty' delicacy to another sophont.
4. Sophont is a necessary medicinal source for another sophont that must be cooked and ingested orally.
5. Sophont ingests another sophont because: aphrodisiac, psionic booster, physical/mental booster, illegal drug effects...
6. I'm sure there are other reasons.
It looks like I didn't accurately remember my own post.
I also had the thought of
7. Main ingredient for Anagathics.
This gets the same answer as 4/5. If a sophont needs to be harvested for this, there would either be a donor program where people leave their bodies to science or science would, perhaps not immediately, but pretty quickly, find a less controversial substitute. If people tried this against humans, it would very quickly become a HFY story as humans fight back.