Soldiers who have killed in war at close quarters talk about how it affects them today.
Those who've committed the ultimate deed reflect on how it has – or hasn't – lived with them in their minds.
They talk frankly about their feelings before, during and after.
And they reflect on whether humans are "natural" killers or whether they have to be trained to go against their instinctive repulsion.
(Image: A British soldier on patrol in Basra Province, Credit: Press Association)