The uncorruptible cop is the Christ-figure—the one who walks through hell and returns with the truth. They are not naive (the naive cop gets eaten in Act 2). They are vigilant . They know their own capacity for evil, and they build a wall of asceticism, logic, or pain to keep it out.

By the end, the Cop (Mills) is tricked into committing the sin of Wrath (murdering Doe to avenge his wife). The badge does not protect him from damnation. Fincher argues that to hunt the Devil, the Cop must enter the Devil’s logic. Once inside, the Cop becomes indistinguishable from the monster. Mills’ final walk away from the crime scene is the walk of a fallen soul. The uniform is just a shroud. In The Dark Knight (2008), Batman is technically a vigilante, but he functions as the ultimate Cop. Harvey Dent is the White Knight—the incorruptible district attorney. The Joker is chaos incarnate. But the Joker wins not by killing Dent, but by corrupting him. Two-Face is the Devil Cop: a man of law who now uses a coin (fate, chance, the demonic random) to decide who lives or dies.

Why? Because Rust Cohle has a counter-weight: He knows the Devil is real, and he hates him.

When the Cop falls, he doesn't just commit a crime. He annihilates the difference between law and lawlessness. He proves that hell is not a place beneath the earth. Hell is a precinct where the lights are on, the coffee is hot, and no one is watching.