
Fray's tutelage aside, there is nothing specifically that says Dark Knights must have a Dark Side and go through everything verbatim the Warrior of Light does. It's reasonable enough to simply be someone taking a more violent approach to justice than the law allows without having to overcome inner darkness. The title is used to describe the things people do with the power, not the people who necessarily wield it, and there's no way anyone can convince me that it's only possible to kill corrupt clergymen with My Chemical Romance on repeat in the background.