A young cowboy from Texas who joined the elite US Navy Seals became the most deadly sniper in American history. In a book published this month he provides an unusual insight into the psychology of a soldier who waits, watches and kills.

As US forces surged into Iraq in 2003, Chris Kyle was handed a sniper rifle and told to watch as a marine battalion entered an Iraqi town.

A crowd had come out to greet them. Through the scope he saw a woman, with a child close by, approaching his troops. She had a grenade ready to detonate in her hand.

"This was the first time I was going to have to kill someone. I didn't know whether I was going to be able to do it, man, woman or whatever," he says.

"You're running everything through your mind. This is a woman, first of all. Second of all, am I clear to do this, is this right, is it justified? And after I do this, am I going to be fried back home? Are the lawyers going to come after me saying, 'You killed a woman, you're going to prison'?"

It's killing that is very distant but also very personal - I would even say intimate Neta Bar, Anthropologist

But he didn't have much time to debate these questions.

"She made the decision for me, it was either my fellow Americans die or I take her out."

He pulled the trigger.

Kyle remained in Iraq until 2009. According to official Pentagon figures, he killed 160 people, the most career sniper kills in the history of the US military. His own estimate is much higher, at 255 kills.

According to army intelligence, he was christened "The Devil" by Iraqi insurgents, who put a $20,000 (£13,000) bounty on his head.

Married with two children, he has now retired from the military and has published a book in which he claims to have no regrets, referring to the people he killed as "savages".

Job satisfaction

But a study into snipers in Israel has shown that snipers are much less likely than other soldiers to dehumanise their enemy in this way.

Image caption Chris Kyle killed an estimated 40 people during the second battle of Fallujah in 2004

Part of the reason for this may be that snipers can see their targets with great clarity and sometimes must observe them for hours or even days.

"It's killing that is very distant but also very personal," says anthropologist Neta Bar. "I would even say intimate."

She studied attitudes to killing among 30 Israeli snipers who served in the Palestinian territories from 2000 to 2003, to examine whether killing is unnatural or traumatic for human beings.

She chose snipers in particular because, unlike pilots or tank drivers who shoot at big targets like buildings, the sniper picks off individual people.

What she found was that while many Israeli soldiers would refer to Palestinian militants as "terrorists", snipers generally referred to them as human beings.

Tale of a Soviet sniper There were about 20 gunmen escorting a convoy and one of them was unlucky enough to get in the sight of my scope. The distance was about 300m, almost nothing for a sniper. A few seconds later I saw him lying motionless. In the heat of the moment my only thought was to shoot more and more. I saw the figures rushing in panic and trying to hide. We killed all of them, except three or four who were wounded and captured. Afterwards I blamed myself for not being cool-headed enough. I thought that if I had been calmer, I would have killed more enemies. We were proud of ourselves, but now I am ashamed. If I was asked today, I would say it's very hard to kill, but more than 20 years ago I was too young. Ilya Abishev (pictured second from right) fought in Afghanistan in 1988

"The Hebrew word for human being is Son of Adam and this was the word they used by far more than any other when they talked about the people that they killed," she says.

Snipers almost never referred to the men they killed as targets, or used animal or machine metaphors. Some interviewees even said that their victims were legitimate warriors.

"Here is someone whose friends love him and I am sure he is a good person because he does this out of ideology," said one sniper who watched through his scope as a family mourned the man he had just shot. "But we from our side have prevented the killing of innocents, so we are not sorry about it."

This justification - which was supported by friends, family and wider Israeli society - could be one reason why the snipers didn't report any trauma after killing, she suggests.

"Being prepared for all those things that might crack their conviction, actually enabled them to kill without suffering too much."

She also noted that the snipers she studied were rational and intelligent young men.

In most military forces, snipers are subject to rigorous testing and training and are chosen for aptitude. In the UK, they complete a three-month training course, with a pass rate of only one in four.

Every person I killed I strongly believe that they were bad Chris Kyle

The US marine sniper course is one of the hardest training courses in the military, with a failure rate of more than 60% and a long list of prerequisites for recruits, including "a high degree of maturity, equanimity and common sense".

Research in Canada has also found that snipers tend to score lower on tests for post-traumatic stress and higher on tests for job satisfaction than the average soldier.

"By and large, they are very healthy, well-adjusted young men," says Peter Bradley at the Royal Military College of Canada, who is studying 150 snipers in Afghanistan. "When you meet them you're taken by how sensible and level-headed they are."

Don't tell your wife

But both the Israeli and the Canadian studies only spoke to snipers who were still on active duty. Neta Bar suspects many of them could experience problems in years to come, after they return to normal society.

What causes trauma? The US Department of Veterans defines three sources of war-related trauma: being at risk of death or injury, seeing others hurt or killed and having to kill or wound others. Snipers suffer less of the first but arguably more of the latter two.

"A sniper is exposed to less unknown danger, has better reach and sees more," says a former infantryman and spokesperson for the UK army. "They won't be exposed to as much trauma as a dismounted, normal-operative infantryman."

When former Soviet sniper Ilya Abishev fought in Afghanistan in 1988 he was immersed in Soviet propaganda and was convinced what he was doing was right.

Regret came much later. "We believed we were defending the Afghan people," he says. "Now I am not proud, I am ashamed of my behaviour."

For police snipers, who operate within normal society rather than a war zone, doubts, or even trauma, can arise much sooner.

Brian Sain, a sniper and deputy at the sheriff's department in Texas, says many police and army snipers struggle with having killed in such an intimate way.

"It's not something you can tell your wife, it's not something you can tell your pastor," says Mr Sain, a member of Spotter, an American association that supports traumatised snipers. "Only another sniper understands how that feels."

But for the US's deadliest sniper, remorse does not seem to be an issue.

"It is a weird feeling," he admits. "Seeing an actual dead body... knowing that you're the one that caused it now to no longer move."

But that is as far as he goes.

"Every person I killed I strongly believe that they were bad," he says. "When I do go face God there is going to be lots of things I will have to account for but killing any of those people is not one of them."

Chris Kyle was interviewed by Outlook for the BBC World Service. Listen to the interview here.

Chris Kyle's book is called American Sniper.