Possibility: Normal harry and his Dark Side have now merged (or Harry has lost his restraint to the extent the distinction is irrelevant). This new Harry has all his abilities and none of his previous restraint and is effectively a new person, with the expressly stated intention of changing the world that now exists to the extent it is effectively destroyed.

Secondary possibility: The current 'world' will end because Harry is going to somehow turn back time and destroy thus timeline in its entirety.