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Spoilers for the ending of Martin Scorsese’s 2006 film. Billy Costigan means to arrest Colin Sullivan, not execute him: when Sullivan says, “Just kill me,” Costigan replies, “I am killing you,” meaning he is ending Sullivan’s freedom by bringing him in. Scorsese later said Costigan was “going by the book” and “trying to be a good man.” The plan collapses when Trooper Barrigan kills Costigan; Sullivan kills Barrigan, but Sean Dignam eventually finds Sullivan at home and shoots him. The final rat and Massachusetts State House image leaves the story with a darkly comic, ominous coda about betrayal—not a new plot clue.
What happens at the end of The Departed?
The ending brings the film’s two infiltrations together. Colin Sullivan has been passing information to mob boss Frank Costello from inside the state police; Billy Costigan has been working undercover inside Costello’s organization. Once Costigan discovers Sullivan’s connection to Costello, he confronts him and tries to take him into custody.
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Sullivan asks Costigan to kill him. Costigan’s answer—“I am killing you”—means he intends to end Sullivan’s life as a free man by arresting him and exposing him through the law. Scorsese later explained the line plainly: “He was going by the book.” He added, “He’s trying to be a good man.” TIME reported Scorsese’s comments on October 15, 2019, recounting an interview with Entertainment Weekly.
Before Costigan can bring Sullivan in, Trooper Barrigan shoots him in the elevator. Brown is also killed. Sullivan then kills Barrigan, making the apparent mole look dead and buying himself time. He attends Costigan’s funeral and returns home, where Dignam is waiting. Dignam shoots Sullivan, and the film ends with a rat crossing a railing against the State House in the background. The sequence is also recapped by Film Colossus.
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Why does Costigan say, “I am killing you”?
Costigan is not agreeing to shoot Sullivan. He is saying that arrest and exposure will destroy Sullivan’s ability to live freely as a police officer and a protected informant. Scorsese’s explanation settles the key ambiguity: Costigan is trying to follow procedure, not carry out an execution. The line is blunt and threatening, but its purpose is to make clear that Sullivan’s choices have consequences within the law.
Why does Dignam kill Sullivan?
Dignam has reason to hate Sullivan for his part in the corruption and deaths surrounding Captain Queenan. By the time Dignam reaches Sullivan, the official process has failed to deliver a hearing or trial: Costigan is dead, Sullivan has escaped immediate exposure, and Dignam chooses to kill him himself.
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That act gives Sullivan a final consequence, but the film does not show a lawful adjudication. Calling it simply “justice” misses the tension the ending leaves behind: Dignam may be avenging the dead, yet he also acts outside the system Costigan tried to use. This is an interpretation of the events, rather than a stated explanation from the filmmakers.
What does the final rat mean?
The rat works on more than one level, and the filmmakers’ comments do not reduce it to a single mandatory meaning.
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A literal echo of the human “rats”
Throughout the film, “rat” refers to an informant or traitor. The final animal echoes that language after the human informers have been exposed or killed. It is not shown to be Dignam, another surviving informant, or a clue to a continuation of the plot.
The State House and Colin’s idea of success
Scorsese said he connected the rat’s reveal to the Massachusetts State House’s gold dome and to Beacon Hill, which represented the “top of the world” to Colin. The juxtaposition invites a reading of corruption beside civic authority, especially after the story has shown wrongdoing inside law enforcement. That is a synthesis of the image and the plot, not a meaning the film spells out directly.
Gangster-film tradition, paranoia, and a joke
Scorsese connected the composition to classic gangster-film endings such as Scarface, Little Caesar, and White Heat, and described the image in terms of paranoia and betrayal: no one knows who can be trusted. He also called it “a comment from the filmmakers on the subject matter.” Screenwriter William Monahan recalled first imagining the rat as a little joke after the bloody ending, while adding: “There’s also the idea of the rat behind the wall of Colin’s supposedly perfect world.” Their comments support an ending that is both darkly comic and ominous. PopEntertainment’s interview with the filmmakers was published February 26, 2007, and updated February 26, 2025; Scorsese’s comments on the State House also appear in Blackfilm’s 2006 press conference coverage.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does the ending restore order?
No. Sullivan is dead, but Costigan is dead too, and the film does not show Sullivan being tried or the police institution being repaired. Dignam’s killing closes the immediate story of Sullivan’s escape, yet it does not undo the failures that allowed the corruption to flourish. The rat’s final appearance leaves the image of betrayal in place rather than offering a clean resolution.
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