The Hurt Locker

The Hurt Locker opens with a quotation from War Is a Force That Gives Us Meaning, a best-selling 2002 book by Chris Hedges, a New York Times war correspondent and journalist: "The rush of battle is a potent and often lethal addiction, for war is a drug."[2][3][4]
Sergeant First Class William James (Jeremy Renner), a battle-tested veteran, arrives as a new team leader of a U.S. Army Explosive Ordnance Disposal (EOD) unit in the Iraq War,[5][6] replacing Staff Sergeant Matthew Thompson (Guy Pearce), who was killed by a radio-controlled 155mm improvised explosive device (IED) in Baghdad. His team includes Sergeant J.T. Sanborn (Anthony Mackie) and Specialist Owen Eldridge (Brian Geraghty).
James' maverick methods and attitude lead Sanborn and Eldridge to consider him reckless, and tensions mount. When they are assigned to destroy some explosives in a remote desert area, James returns to the detonation site to pick up his gloves. Sanborn openly contemplates killing James by "accidentally" triggering the explosion, making Eldridge very uncomfortable, but Sanborn does nothing.
Returning to Camp Victory in their Humvee, the team encounters five armed men in traditional Arab garb standing near the men's Ford Excursion, which has a flat tire. After a tense encounter, the men reveal themselves to be private military contractors and British mercenaries. They have captured two prisoners featured on the most-wanted Iraqi playing cards. The entire group suddenly comes under fire, and when the prisoners attempt to escape in the confusion, the leader of the mercenaries (Ralph Fiennes) remembers the bounty for them is "dead or alive" and shoots them. Enemy snipers kill three of the mercenaries, including the leader. Sanborn and James borrow a Barrett .50 cal to dispatch three attackers, while Eldridge kills a fourth.
During a raid on a warehouse, James discovers the body of a young boy, in which a live bomb has been surgically implanted. James believes it to be "Beckham" (Christopher Sayegh), an Iraqi youth he had previously befriended. During evacuation, Lieutenant Colonel John Cambridge (Christian Camargo), the camp's psychiatrist and a friend of Eldridge's, is killed in an explosion; Eldridge blames himself for the Colonel's death. Later, James leaves the military compound seeking revenge for Beckham and breaks into the house of an Iraqi professor, but his search reveals nothing and he leaves.
Called to a petrol tanker detonation, James decides on his own to hunt for the insurgents responsible, guessing they are still in the immediate area. Sanborn protests, but when James heads out, he and Eldridge reluctantly follow. After they split up, insurgents capture Eldridge. James and Sanborn rescue him, but accidentally shoot him in the leg. The following morning, James is approached by Beckham, who James believed was dead. The young boy tries to play soccer with James and sell him more DVDs, but the soldier walks by without saying a word. Before being airlifted for surgery elsewhere, Eldridge angrily blames James for his injury.
James and Sanborn's unit is called to another mission in their last two days of their rotation. An innocent Iraqi civilian man has had a bomb vest strapped to his chest. James tries to cut off the locks to remove the vest, but there are too many to undo in the time available before the bomb will detonate. He has to abandon the man, who is killed when the bomb explodes. Sanborn is left distraught by the man's death. He confesses to James that he can no longer cope with the pressure, and he wants to return home and have a son.
After Bravo Company's rotation ends, James returns home to his ex-wife, Connie (Evangeline Lilly) and their infant son who both still live with him in his house. However, he is bored and disconnected from routine civilian life, with its ordinary tasks of shopping at the supermarket and family dinners. One night, James confesses to his son that there is only one thing that he knows he loves. Shortly thereafter, he starts another tour of duty serving with Delta Company, U.S. Army EOD unit as they are starting their 365-day rotation.

Question: Who does James decides on his own to hunt for the responsible?
Answer: Insurgents