Bruce Wayne: You know that day that you once told me about, when Gotham would no longer need Batman? It’s coming.
Rachel Dawes: Bruce. You can’t ask me to wait for that.
Bruce Wayne: It’s happening now. Harvey is that hero. He locked up half of the city’s criminals, and he did it without wearing a mask. Gotham needs a hero with a face. [ The Dark Knight]
And here is the masterpiece:
Natascha: But this is a democracy Harvey…
Harvey Dent: When their enemies were at the gates, the Romans would suspend democracy and appoint one man to protect the city. It wasn’t considered an honor, it was considered a public service.
Rachel Dawes: Harvey, the last man who they appointed the Republic was named Caesar and he never gave up his power.
Harvey Dent: Okay, fine. you either die a hero or you live long enough to see yourself become the villain. [The Dark Knight]
Harvey Dent, the white knight, the hero of Gotham city eventually becomes Harvey Two Face, a criminal.
Building a good society needs long term efforts. Heroes don’t live that long and if they do, they might as well turn into villains.