If you search for 亡くなる (なくなる) in a proper Japanese dictionary like the Daijisen (
http://dictionary.goo.ne.jp) it says this:
(亡くなる)「人が死ぬ」の婉曲的な言い方。
My translation: 亡くなる is a euphemistic way of speaking of a death of a person.
Definition of a euphemism: "a mild or indirect word or expression substituted for one considered to be too harsh or blunt when referring to something unpleasant or embarrassing."
So you'd use なくなる when speaking carefully about someone passing away recently, to avoid being too blunt about it.
If it is just something like an insect dying, then for this you wouldn't say なくなる. It would be weird to say something like "the snail I stepped on has passed away", lol, you would just bluntly say it died (死んだ).
September 4, 2015 at 8:19am