Death Sentence

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Nick Hume (Kevin Bacon) his wife Helen (Kelly Preston), and his two sons, Brendan (Stuart Lafferty) and Lucas (Jordan Garrett), watch Brendan's hockey game. While Nick and Brendan drive home from the game, they talk about the latter's potential future as a professional hockey player. They make a quick stop at a gas station in a very bad part of town. During an ostensible robbery of the gas station, Joe Darley (Matt O'Leary), a new initiate and younger brother to the gang leader, slices Brendan's throat open with a machete. Nick attempts to ambush the thugs, managing to pull off Joe's mask and see his face, but Joe escapes just to get hit by a car. Nick rushes Brendan to the hospital, but Brendan dies from a major blood loss.
Nick identifies Joe in a line-up, but while meeting with the district attorney, he becomes angry at the DA for presenting a strong case that the defense will cut a deal of being in jail for 3 to 5 years. The DA explains that the only witness is Nick, there were no surveillance cameras, and the defense could gain sympathy for Darley. At a pre-trial hearing, Nick recants his identification so that Joe will go free. He follows the gang to see where they hide out, later returning and stabbing Joe upon finding him alone. The next morning, the gang leader, Billy (Garrett Hedlund), wants revenge. After quickly discounting members of other gangs as culprits, one of his gang members says that his sister saw a man in a suit that night. Confirming it was Nick from a picture published in a newspaper, and seeing where he works, they ambush him the next day on the street. After chasing him through town, they stop at the top of a multi-story parking garage where Nick gets into a fight with one of the members and traps him in a car and sends him over the edge of the lot.
The next day, one of the gang members then arrives at the office where Nick works to deliver his suitcase that he dropped during the chase. Nick calls a phone number found in the case, which belongs to the gang leader Billy. Billy warns that Nick has bought a "death sentence" for his family, and reveals that Joe Darley was his brother. Nick immediately calls the police detective assigned to Brendan's case, Jessica Wallis (Aisha Tyler), who is already aware of what Nick started. She grants Nick's family police protection and issues APBs on Billy and his gang. That night, the officers at Nick's house are stealthily killed, but by the time Nick realizes, he finds the gang members have already made their way inside. They attack and subdue Nick, then drag Helen and Luke downstairs to shoot them all.
Nick and Luke survive, but Helen does not. After Detective Wallis gives a brief speech that wars are never settled, she lets Nick pay a short visit to his now-comatose son, where he apologizes for not being a better father. Nick escapes from the hospital to go after the remaining gang members, obtaining guns from a black market gun dealer known as Bones (John Goodman), Billy's father, who accepts the fact that Nick wants to kill his son. Nick tracks down Heco, another member of the gang, and interrogates him about where the other members are, learning their lair is an abandoned mental hospital they call "The Office." He forces Heco at gunpoint to call Billy's cell phone, and executes Heco while Billy is listening. Bones confronts Billy and criticizes him, but Billy kills him. Nick heads to "The Office" to kill the remainder of the gang. After an intense shootout, he and Billy encounter and seriously wound each other in the hospital chapel. Sitting on the same pew, Billy claims that he turned Nick into a vicious cold-blooded killer, just like him. Nick pulls out one of his guns and asks Billy if he is "ready". Nick kills Billy at the end of the scene.
With his family now avenged, Nick returns home and starts to watch home movies and await his inevitable arrest. Detective Wallis comes to arrest Nick but first informs him that his son has miraculously moved and will now live. Nick becomes relieved and looks back to the TV, which shows Luke, Helen, Nick, and Brendan singing on the couch. Nick's final fate remains unknown. In the extended version of the film, Nick succumbs to his injuries.

Question: Who is the gang's leader?
Answer: Billy