Ok, I see, I think I understand this a bit better now. About the "desires" thing, that wasn't related to your game world directly. That was just an allegory in regards to a slightly similar (but not identical) concept. Or rather, what I thought to be a similar one.
Anyway, if I were to create a world (as a god), I would probably make it a tabula rasa. So in essence, everyone would truly choose their path, I would not predetermine anything for them. Becoming "good" or "evil" (or rather. something in between) would be a choice of those beings themselves, while influenced by their surroundings - so, much like our world. They wouldn't even have to have ethics, if it's not in their nature. And then I would just watch how that universe evolve without interfering (unless I get bored ;).
Anyway, on a logical level, your comparison between growing up and eating and spiritual growth makes sense (both being absolutely natural in their respective universes). Of course, if one were to artificially stop their growth or stop eating and drinking, or start hurting themselves in our world, we would consider that a mental illness as well. So I get that analogy.
I just never made that connection in my mind while playreading, especially since you depicted the "infected" as rather normal individuals. Ah, and also because Diane seemed to be a rather murderous and mad person to me at a certain point in the game.
Anyway, thanks for taking the time to explain this a bit better!