Batman may be the darkest hero.
Although he saves Gotham at the end of the day, he also instills fear within those who would stand against him (the reason why Bane is intrigued with him to begin with). This is because of the fear that had always been instilled within him:
As a little boy, Bruce Wayne's parents were both killed in front of him. This gave Bruce the understanding of how short the arm of the law really is. He knew that he could do more to stand against it.
This motivation not only showed him how not to live and what not to do, but also what to do. Bruce was familiar with the terror and helplessness that the thug had forced upon him as a boy and he knew that he could use this same tool to stop the men who were like the one that had hurt him. He took on the mantle of the Bat, an animal that prowls the night and horrifies many (including the young Bruce himself).
Almost all of the comic book devotions so far have been based on the villain instead of the hero. It would seem to be much easier to relate to the enemy than the savior. Perhaps it is because of sin, due to our own human state, or because we as a race have often taken the side against God instead of standing with Him.
Batman is just like that. Although he might not be the "villain" of the story, his darkness is easy to relate to. We have all witnessed the power fear and might have even wished to make others fear us. We have all lost people or things dear to us and have gone to almost any means to get them back, or at least avenge them. We have given into the darkness in order to achieve something that seems to be right.
This darkness is one of the strongest powers known to man. Making him crave even more than he had started with or could ever need. And, clouding his judgment so that he might never again be able to see the light.
How has Batman overcome this darkness that he is filled with?
It is because he is not only Dark, he is also a Knight.
He has more than just the guidance, reflection, and direction that understanding the darkness and justice can give. He has hope.
Besides knowing all of the evil in the world and his undying need to stop it, he knows what the world should be like. He doesn't merely aim to stop what is wrong, he wants to make it right. Instead of bringing the world from bad to neutral, he wants to make it good like any true knight would do.
He wants to take what is evil and not only be rid of it, but to redeem it and make it pure and good.
This is what Christ has done for us. He knew the evil, the Law, and how to keep it. But, more-importantly, he knew the good news, the Gospel. He knew what he had come to do, redeem the world. He had hope and he fulfilled his hope for us through his death and resurrection; our purification.
He has become our Knight who leads us away from the Dark.