Throughout the film, Martin Scorsese used Xs mostly shown in the background to mark characters for death; examples include shots of Costigan walking through the airport while talking to Sgt. Dignam, Queenan falling to his death (on the building’s glass windows as Queenan falls to the ground), and Sullivan in his office discussing the flow of information with Costello (the X is created by the light shining through the window). This is a homage to Howard Hawks’ classic film Scarface (1932).
“When I was your age they would say we can become cops, or criminals. Today, what I’m saying to you is this: when you’re facing a loaded gun, what’s the difference?”