Stephen Evans examines how soldiers are taught to kill and the psychological effects of becoming a killer.

"The hardest time is doing it the first time. After I pulled the trigger for the very first time in Basra when I was 19 years old, it got a lot easier after that".

Ben Close, a former member of the Coldstream Guards, talks about the first time he killed in battle. It's an experience that transformed him. "I was like a time bomb ticking waiting to go off. I was ready to kill in an instant".

Stephen Evans examines how soldiers are taught to kill and the psychological effects of becoming a killer.

At Sandhurst, the elite military establishment where the British Army has trained officers since 1812, the recruits are put through their paces. In a simulator - reminiscent of a very big, realistic video game - Stephen sees how life and death decisions about taking human life are made - the moral dilemmas that have to be juggled in the heat of tense, frightening and chaotic battle.

Stephen explores why talking about killing is the great taboo in armies across the world. Lieutenant Col Pete Kilner, an officer in the United States Army, says "As a profession of arms, we recruit people to kill, we train people to kill, we make the orders for people to kill, yet after the fact we don't talk about killing.

Stephen asks if this is partly to blame for the long-term effects killing has on many soldiers.

Andy Wilson, an ex-SAS soldier, was commended for bravery in Afghanistan. But he was unable to cope with the experience of taking another man's life. "I'd killed him. Whenever I closed my eyes, it kept flashing back to me. I basically stayed awake for four days. On the fifth day I collapsed because I'd had no sleep. And then I'd be awake for another four days. It is torture at night, pure torture" Andy's nightmares and flashbacks led to him being diagnosed with Post Traumatic Stress Disorder.

The programme combines gripping first-hand testimony from those who have been on the front line, with powerful archive and contributions from the experts who are wrestling with the theory and practice of killing in warfare.

Producer: Adele Armstrong.