light and darkness are always at a balance one way or another regardless of how the story progresses... just like Ruran said, the more light there is,there is a shadow just as great. The good news is that light has no inheirent flaws although at the cost of balance being kept, light is just as strong as darkness and always will be. So when light is really strong, so is darkness. And although the concept of 'good triumphing over evil' is pleasing to the human mind, it would go against our own concept of balance if that happened. and I do believe xehanort is corrupt because if he had the same standpoint as Ruran did on the concept of balance... he would see that there was balance in the worlds regardless of what he trid to change it an if he did there'd be no point to KH's plot if there was no villain.