In the Batman comic series--and in the 2008 film Batman: The Dark Knight--Harvey Dent is an ideal district attorney. He's a tough-on-crime prosecutor who won't be intimidated or bribed by the crime bosses running Gotham. Given that law enforcement (i.e., Batman and Commissioner James Gordon) need a representative on the legal side of things, Dent's role is crucial.
Alas, such an alliance cannot last. Dent is disfigured when a crime boss splashes his face with sulfuric acid, leaving half of his face exposed with gruesome scarring.
Dent becomes the enemy: "Two-Face." His face reflects his split personality, his crisis of conscience: sometimes good, sometimes evil. He carries a coin with him, and he uses the coin to render decisions for him: good or evil, heads or tails, the choice left to fate.
I confess that I'd love to read the story of the Gotham Bar Association revoking Dent's bar license for criminal activity... but that's probably only going to be of interest among those for whom the Fictional Attorney of the Month is of interest. Until then, the ethics will remain an underground, niche storyline.