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”
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”.
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.”
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.”