That can would be Marco asking Mexican mob boss Fausto - well played by both Demian Bichir and Ramon Franco - if Fausto would take care of a little something for Marco: killing Hastings/Tate in prison. Marco is of course entirely justified in doing this, seeing as how the sociopath Tate killed Marco's boy Gus - but the request and Fausto's agreement still make Marco an entirely different character for next season.
Marco has now joined the ranks of iconic anti-hero cop Vic Mackey of The Shield and, more recently, Frank Agnew of Low Winter Sun. Mackey is one of the most compelling characters ever to have been seen on the television screen - right up there with Walter White and Tony Soprano - and though Agnew is a long way from that, the territory of a cop killing another cop is rich with dramatic possibilities.
Marco, of course, is asking Fausto to kill a special kind of cop. Not someone investigating his corruption, as was the case with Mackey, or someone who might have killed the woman that he loves, as is the case with Agnew. Neither of those two motivations are as powerful and justified as the grievance Marco has against Tate - his former partner, who goes on to kill Marco's son.
Further, what's noteworthy about this turn of events in The Bridge is that we've come to know Marco as an honest cop - we've seen him working honestly and courageously at his job all season. In contrast, Mackey and Agnew killed their respective cop in the very first episodes of their series. This means we never got to know them as honest cops, before they went over to the dark side, and we can only guess what they were like before they took their plunge into the abyss.
Marco has been a great character to watch all season - as has Sonya and Hank - and I'm looking forward to more in 2014.
See also The Bridge Opens Brooding and Valent ... The Bridge 1.2: A Tale of Two Beds ... The Bridge 1.6: Revelations ... The Bridge 1.7: A Killer and a Reluctant Professor ... The Bridge 1.8: Some Dark Poetic Justice ... The Bridge 1.9: Trade-Off ... The Bridge 1.10: Charlotte's Evolution ... The Bridge 1.11: Put to the Test