I'm of the belief that we don't know the whole story of Zodiark/Hydaelyn yet. While Elidibus is certainly ruthlessly determined, especially if it is confirmed that he was the one who gave Urianger the second auracite, but I feel as if SE won't go pure "dark is evil, light is wonderful and perfect and best and the only way."

I think it's too early to be saying that Hydaelyn is pure good and everything Elidibus does is for the sake of pure evil.