It's a pretty grim thought and is a step towards trying some pretty blearghsome toppings too! 🙁 On a basic level, if it wouldn't poison you and could nourish you, it's at least plausible, even if it is a gross thought. On an ethical level you would have to disclose what it is you are making your burgers with (as Doublemeat would have to divulge their veggie truth in real life) and I think most people would be put off 'demon burgers'. "Sewer rat may taste like pumpkin pie, but I wouldn't know..." 😀
I mean really, meat is meat, and it seems mostly an arbitrary determination by society of what is frowned upon to eat and what is 'correct', probably dictated mostly by things like the prevalence, ease to farm, per animal yield etc. I don't know. But the sentience of the creatures and their ability to object as much as to fight back plays a part I'd have thought too in what could be put under the ethical considerations. So there would probably be some degree of thought given as to whether the demons were threats to humans, so whether it was already the case that they were killed for survival rather than just being hunted for food. We raise/kill sentient creatures for food all the time, but whether a species of demon could be classed the same, as potential livestock, would be questionable for sure. If they were not a threat and they were also fully capable of communicating too (say Clem for example) then I think you are definitely into very bleak areas of ethics to start hunting/farming them as a food source. Particularly when there are plenty of other alternatives available.
Not sure I'm still in the mood for tea now. 🙁