I'll accept that willpower means something like the conscious mind trying to reign in the subconscious. But when you use that to defend the "ethics as willpower" view, you're assuming that the subconscious usually wants to do immoral things, and the conscious mind is the source of morality.

On the contrary, my subconscious is at least as likely to propose moral actions as my conscious. My subconscious mind wants to be nice to people. If anything, it's my conscious mind that comes up with evil plans; and my subconscious that kicks back.

I think there's a connection with the mythology of the werewolf. Bear with me. Humans have a tradition at least 2000 years long of saying that humans are better than animals because they're rational. We characterize beasts as bestial; and humans as humane. So we have the legend of the werewolf, in which a rational man is overcome by his animal (subconscious) nature and does horrible things.

Yet if you study wolves, you find they are often better parents and more devoted partners than humans are. Being more rational may let you be more effective at being moral; but it doesn't appear to give you new moral values.

(I once wrote a story about a wolf that was cursed with becoming human under the full moon, and did horrible things to become the pack alpha that it never could have conceived of as a wolf. It wasn't very good.)