The horrors of working in bomb-torn Baghdad contributed to a father's mental state on the day he killed his son, a court heard.

Andrew Wragg's experiences as a bodyguard in Iraq, problems with his marriage and his son's worsening illness produced an "abnormality of mind" when he placed a pillow over ten-year-old Jacob's face.

Wragg, a 37-year-old former SAS soldier, suffered reaction to severe stress and adjustment disorder, said Dr Nicholas Wright, a consultant forensic psychiatrist.

Dr Wright was giving evidence on day seven of the trial in support of the defence case that Wragg did not murder Jacob but committed manslaughter while suffering from diminished responsibility.

Wragg killed Jacob last July at the family home in Henty Close, Worthing, just after returning from a three-month tour in the Iraqi capital.

Lewes Crown Court heard Wragg was earning £80,000 a year as a private bodyguard, a job he took on to pay off debts and escape from an "increasingly intolerable" situation at home.

Dr Wright interviewed Wragg, just days after Jacob's death, at Lewes Prison where staff described Wragg as shell shocked.

Wragg told the doctor of his experiences in Baghdad, where he had lived in the "green" protected zone which was regularly mortar bombed.

He drove a "thin-skinned" vehicle, not armoured, and escorted people to meetings.

He said: "He was running the risk of being shot, a suicide bombing, or being taken hostage ... it caused incredible strain and extreme stress.

"Two of his colleagues were injured, one seriously, and an Iraqi driver was killed.

"He was driving regularly to the airport on the most dangerous road in the world, several times a day."

Dr Wright said the Army was extremely concerned all personnel in Iraq were "suffering from something abnormal" and requiring them all to be assessed for the same condition.

He said: "There are reports that a great number of people are coming back with adjustment disorders."

Dr Wright said when Wragg returned to his home in Worthing he acted like a lot of homecoming soldiers.

He said: "The first thing he did was drink heavily to relax."

Wragg, he said, was met with marital tension, the realisation that Jacob had deteriorated and there was no progress with social services in finding his son residential care.

Dr Wright said: "I don't think Mary appreciated the effect Iraq had on Andrew Wragg."

Another major contributing factor to his stress disorder was the termination of his wife's seven-month pregnancy.

Their unborn son, Henry, had been diagnosed with the same degenerative Hunter syndrome Jacob suffered and the couple went through the agony of watching the foetus being killed.

Dr Wright said: "I don't think the trauma has left Mr Wragg and probably Mrs Wragg as well. It was utterly horrifying."

Jacob's condition made Wragg worse.

The boy was deaf, unable to speak, incontinent, was coughing and suffered frequent angry outbursts.

Dr Wright said: "Looking after an extremely disabled child often causes marriages to founder.

"It causes enormous tensions between the parents."

Days before the killing, Wragg had cried while watching the film Lorenzo's Oil, about a couple with a disabled child.

Dr Wright said: "Crying was unusual for him. He is not an emotional person. This indicated not only was he tense but quite depressed.

"He found the film profoundly depressing."

Dr Wright said all Wragg's stresses were "building up to breaking point".

Just hours before killing his son, Wragg said he looked into Jacob's eyes and could see the boy wanted him to do something.

Dr Wright said Wragg was verging on delusion but it was a defining moment, sufficient to trigger the thought that the situation was intolerable and something had to be done.

Wragg drank heavily that night but, Dr Wright said, remained sober because he was so tense. Wragg sounded calm and collected on a recording of his 999 call to police minutes after the killing.

During the Lewes Prison interview, Wragg told Dr Wright he had little memory of smothering Jacob but recalled how he held the boy in his arms afterwards.

Dr Wright said he found Wragg had no mental illness such as schizophrenia but added: "It seemed to me that at the time he killed Jacob he was suffering from abnormality of the mind.

"It is what is called reaction of severe stress and adjustment disorder, not dissimilar to post traumatic stress disorder."

The disorder often led to victims cracking up or acting irresponsibly and would have substantially impaired his mental ability. Dr Wright said killing Jacob was irrational for Wragg because he had nothing to gain by it - he was planning a holiday to Gran Canaria the week after Jacob's death and he was planning to return to Iraq for more bodyguard work.

Wragg's actions, he said, were those of an over-controlled person who looked normal on the outside but was abnormal on the inside.

Dr Wright stressed: "I am not saying what he did was irrational. I'm saying the compulsion in his mind that made him do it was irrational."

Cross-examined by Philip Katz QC, prosecuting, Dr Wright agreed Wragg said nothing to him during the interview about Mary asking him to kill Jacob or that he had spoken of terminating the boy's life on at least two occasions before the smothering.

Dr Wright said Wragg's disorder, recognised by the World Health Organisation, might last only a day and he agreed with Mr Katz that Wragg showed no signs of the illness during the interview in Lewes prison.

He agreed Wragg was not suffering the disorder now.

But Dr Wright said: "In a funny sort of way the whole thing subsided (after the killing).

"He was no longer tortured by Jacob's suffering. It was ended by Jacob's death.

"He was still depressed and anxious but essentially the turmoil in his mind was over."

The trial is expected to resume on Monday.