Departed: Here's What DiCaprio's Cryptic Final Line Really Meant

It also features one of Leonardo DiCaprio 's most memorable roles, memorable both for DiCaprio himself and for the audiences, that of Trooper Billy Costigan Jr., an undercover agent, who is working as the police's mole in the Irish Mob and trying to figure out who in the police works as the mob's mole, before his own identity is figured out.

And one of Costigan's most memorable scenes is his final one, in which he finally manages to corner and capture the corrupt cop Sullivan on the rooftop of the same dilapidated building where Capt. Queenan (Martin Sheen) perished earlier in the film, thanks to Sullivan's machinations.

As they ride down, Sullivan whines, "just kill me." And Costigan responds with ominous, "I am killing you."

But as immediately after the elevator's doors open Costigan is shot and killed by another corrupt cop, trying to help Sullivan, the viewers are left to question what exactly Costigan meant by those words. Did he mean "killing" metaphorically, in the sense that by arresting Sullivan he's destroying his life, career and ambitions, or did he literally plan to execute Sullivan for his long list of crimes, after all?

Martin Scorsese, however, eventually cleared up the meaning of that line in an interview with Entertainment Weekly and said that Costigan definitely intended to turn Sullivan in.

"He was going by the book," explained Scorsese. "He's trying to be good man."

Which, of course, was the whole point of the scene and its outcome – in a dark world of double agents and mistrust, where it is difficult to say where the police ends and the mob begins, Costigan's attempt to work "by the book" after spending years of his life on a shady infiltration operation was tragically misguided and out-of-place.

Unsurprisingly, in the very end of the movie Sullivan gets his just desserts when another cop, aware of his crimes, but incapable of proving them, simply shoots him vigilante-style.