Aritotle characterises good in functional terms, and so the "good for humans" is at least to some extent different from the "good for spiders". But moral theologians who have drawn on Aristotle (eg Aquinas) clearly think there is a notion of good that extends to god and angels.I personally have never seen any sort of definition set of objective good which was not anthropocentric.
Kant characterises right action in terms of pure reason, so that is certainly not intended to be anthropocentric.
Bentham, Singer and others define good in terms of preference satisfaction and the avoidance of suffering, and that is expressly intended to apply to animals other than humans, as well as humans.
The healing spell doesn't seem to me to raise any more puzzles than (say) an injection of methodone, which is probably not good for me, but is good for someone who is using the drug as part of a treatment for overcoming a heroin addiction.In D&D however you have scads of different intelligent species with quite radically different needs and drives and it seems to me that this must quite explode the entire concept of obective value. Certainly where good is concerned. Even something as apparently universally good as a simple healing spell can be readily demonstrated to be "not good" as it would harm a Xeg-yi, an intelligent non-evil creature, thus being harmful to something which does not oppose good.
The issue of diverse drives etc is interesting, but when I look at mainstream humanoid races in D&D they tend to display less cultural variation than is actually found among human beings as they have actually lived at different times and places across the earth!
Isn't "always evil" a piece of rules text - it tells the GM that demons are almost always evil. It doesn't follow that, in the game, the demons didn't make that choice. (Imagine an entry that said "Soup kitchen volunteers - usually good": it wouldn't follow that in the game those people hadn't chosen to make an effort to help others.)Does a being like a Demon which is "Always Evil" have free will if he cannot choose to do good? And if he does not, how can he be evil, without choice?
To the extent that the standard planar metaphyics of D&D makes demons manifestations of a non-sentient but necessarily destructive and hateful place (the Abyss), and hence does suggest that they have no choice and hence are (in some sense) not really evil, that for me just compounds the incoherence of alignment that I mentioned in reply to Ratskinner.
This is also why I tend to find Planescape quite cynical in its outlook.