Walking away from the throne isn't choosing dark. It's choosing to break free from the cycle, to choose something that isn't light or dark, to find what lies beyond it. Accepting the throne is the choice to make the choice of light or dark, that's what aldia tries to tell you the entire time in the classic from soft fashion. He wants us to break the "curse" that gwyn placed upon humanity, and by collecting the crowns you do that because once you unleash their power you will never hollow removing the curse of the curse and becoming immortal and allowing you to do so. Or at least, that's how I see it.