Harvey Dent: You either die a hero or you live long enough to see yourself become the villain.

He's clearly a man with a mission, but it's not one of vengeance. Bruce is not after personal revenge ... He's much bigger than that; he's much more noble than that. He wants the world to be a better place, where a young Bruce Wayne would not be a victim ... In a way, he's out to make himself unnecessary. Batman is a hero who wishes he didn't have to exist.

Heroes are more than just stories, they’re people. And people are complicated; people are strange. Nobody is a hero through and through, there’s always something in them that’ll turn sour... you’ll learn it one day. There are no heroes, only villains who win.

He thinks he’ll be remembered as the villain in the story”
She snorted.
“But I forgot to tell him,” I said too quietly, opening the door, “that the villain is usually the person who locks up the maiden and throws away the key.”
“Oh?”
I shrugged, “He was the one who let me out.

Heroes are defined by their villains - Batman is nothing if he doesn't have Two-Face.

Why aren't crazy people content to take over, like, one town? It always has to be the whole word. They can't just control maybe twenty people. The have to control everyone. The can't just be stinking rich. The can't just do genetic experiments on a couple unlucky few. They have to put something in the water. In the air. To get everyone.
I was tired of all of it.