Again, the prophecy is NOT about destroying evil or darkness, since that is impossible. Good and evil, light and dark will always be opposed to one another; there is no other conceivable way of life. But in this case, it's not about you have two apples, I also have two apples. We bring balance to the kitchen table. No, it's the Sith seek to destroy good utterly and crush it under oppression, while teh jedi seek to defeat evil's machinations and restore the universe to its natural state, with at least some good present. If the jedi did not seek to merely restore the balance, they would install a regime of their own and conform everyone to be "good", and thus it would lead to eventual corruption and thus evil yet again. Therefore Lucas' point is that good can work for the balance and evil against it.