Shoot 'em Up

At a bus stop in a rough part of town, a drifter named Smith (Clive Owen) sees a pregnant woman fleeing a hitman. Following them into a warehouse, Smith murders the hitman by stabbing him in the face with a carrot. As more thugs arrive, the woman starts to give birth, and Smith is forced to deliver the woman's baby during a shootout. Pursued by head assassin Karl Hertz (Paul Giamatti), the woman is shot and killed, forcing Smith to narrowly escape with the newborn.
Leaving the baby in a park, Smith hopes someone will adopt the child, only for a passing woman to be killed by a shot from Hertz's sniper rifle. Realizing Hertz is trying to kill the baby, Smith saves him, and attempts to leave him with a lactating prostitute named Donna (Monica Bellucci); despite his pleas, she refuses. Hertz arrives at the brothel shortly after and tortures Donna for information, only for Smith to return and kill Hertz's henchmen.
Taking Donna to his hideout, Smith realizes that the baby (who he names Oliver) stops crying when he hears heavy metal music, leading him to conclude his mother lived near a heavy metal club. Followed by Hertz, Smith is then forced to shoot his way out of his hideout, before he and Donna head to a nearby club. Heading above the club, they discover an apartment containing medical equipment and two dead pregnant women; Smith concludes the women were all impregnated with a specific man's sperm so they could birth matching bone marrow donors.
Hiding in a motel room, Smith and Donna are attacked by masked men during sex, Smith realizes his assailants' weapons are all "Hammerson" models unavailable to the public. Before he pursues this clue, Smith takes Donna and Oliver to a war museum, and hides them in a M24 Chaffee tank. Infiltrating the Hammerson factory, Smith witnesses Hertz and Hammerson in conversation about how they do not want the Second Amendment on the right to bear arms repealed by the next President, and notices Hammerson owns a German Shepherd called Duchess. Smith booby-traps the entire facility, allowing him to kill the thugs and escape.
Smith soon notices an article on Senator Harry Rutledge (Daniel Pilon), a presidential candidate campaigning for stricter gun laws. Smith deduces Rutledge has cancer and requires a bone marrow transplant, which is why he had surrogates impregnated with his sperm, and why Hertz and Hammerson want Oliver dead. If the infants die, the Senator will not receive a donation and will be unfit to run as President. Smith tells Donna to leave town, before Smith contacts one of Rutledge's henchmen to request an appointment. Meeting aboard a plane, the Senator confirms Smith's suspicions, only for Smith to notice dog hair on his trousers.
Discerning the hair belongs to Duchess, and that the Senator struck a deal with Hammerson, Smith takes Rutledge hostage, only for Hertz and Hammerson to appear. Escaping from Hertz, Smith kills the Senator and leaps from the aircraft with a parachute. Killing several pursuing henchmen, Smith is shot and, after safely landing, soon collapses due to his injuries. Smith subsequently awakes in Hammerson's mansion. Hertz tortures Smith, breaking his fingers to learn where he sent Donna and Oliver. As Hertz prepares to cut Smith's eyes, Smith manages to break free and kill several thugs. Cornered and struggling to use his gun, Smith places live rounds between his broken fingers and, by detonating them using a fireplace, shoots and critically wounds Hertz. As Smith and Hertz both grab pistols and struggle to kill each other, Smith manages to fire first and kill Hertz.
Boarding a bus with Duchess, Smith soon stops at an ice-cream parlor, where he finds Donna working as a waitress while watching Oliver. The film ends as a group of amateur armed robbers enter the parlor; his hands in bandages, Smith shoots them by using a carrot to pull the trigger.

Question: who attacks Smith and Donna?
Answer: Masked men.