that's not how "the light" works, it is a sentient force, if you have true faith in your cause and wield the light, the light will continue to allow you to use its power, if for whatever reason you stop believing in your cause, then the light will forsake you, in my example of the scarlet crusade, they have tortured many people and killed countless others in the name of the light, something considered "evil" by most peoples standards and yet they are still able to wield the light because of their conviction and believing they are justified in their actions.