Maguire's introductory scene
It's supposed to establish Maguire as an ice-cold killer. He enters an apartment room where a guy who just apparently stabbed a guy who raped the first guy's wife is being led out. Maguire then bribes the other cops into leaving the room, and after he takes the photo, we see that the 'stabbing' victim is still alive and breathing. So Maguire walks up to him and suffocates him with a piece of cloth.
I can't really tell if the guy was actually stabbed or not (seeing as that is blood that trickles out the side of his mouth) or if the idea was that Maguire had come to finish him off and the guy we saw getting led out in handcuffs was in on a staged hit.