WMG / The Hurt Locker

James is actually a very eccentric millionaire.
He got into bomb disposal for the adrenaline rush, but his attitude prevented him from passing the test, so he bribed the instructors.

The kid we see in the end is not the same one as we saw in the beginning.

In other words, Beckham was found dead during that Warehouse raid, and the one in the end was a look-a-like instead of the other way around.

Hurt Locker takes place in an Alternate Universe in which the US Army is Mildly Military.

It's a dangerous place to fight. The rules of ordinance disposal are either much laxer or can be defied without repercussions. They don't even wait until the bullets stop flying to disarm the bombs. Adopt-A-Minefield would be in this 'verse what Greenpeace is in ours.

The military also seems highly undisciplined, and no one seems to be giving orders.

The Hurt Locker is a prequel to 28 Weeks Later

Sergeant William James (Hurt Locker) and Sergeant Doyle (28 Weeks Later) are the same person, and by coincidence, both portrated by Jeremy Renner. His full name is William James Doyle, but dropped the surname Doyle around, presumably because he is estranged with his father à la Angelina Jolie. So hence, he wishes to be called William James only. However, James would later add his surname Doyle because he would reconcile with his dad later on his life. His bomb disposal career would come to an end when his superior's orders him to head to a rage virus infected Britain (Britain is America's closest ally after all!) where he becomes a marksman (he acquires the skill after 3 mercenaries dies at the hands of Iraqi snipers).

As the virus has only hit the UK in 28 Weeks Later, it is safe to assume that his wife Connie and his son are still living in the US and therefore are alive (unless the virus has already reached the US while the American Government at the time keeps it under wraps).

The reason why William James/Doyle is a Sergeant in 28 Weeks Later but a Sergeant First Class (2 ranks higher than Sergeant) in The Hurt Locker is because Sergeant JT Sandborn and/or Specialist Owen Eldridge (he blames James for shooting his leg while he was kidnapped by insurgents) has snitched on James by telling the Generals about James's reckless actions in Iraq and therefore got demoted as a result. On top of that, James has been reassigned to the rage virus infested Britain and got burnt to death by soldiers in NBC suits armed with flame throwers.

James is a Death Seeker
Come on! He pulls off stunts during the bomb defusing like doing it without protective armor, does plans that nearly gets him and his team killed thousands of times and when life at home's boring, he decides to head back.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/TheHurtLocker