A former soldier accused of murdering his dying 10-year-old son "cracked" and became irrational due to combat stress while in Iraq, a court has been told.
Doctors had said Jacob would not live beyond his 20s
Andrew Wragg suffered an "adjustment disorder" after returning to the UK to find his son's degenerative disease was worse, a consultant psychiatrist said.
Dr Nicholas Wright told Lewes Crown Court it had impaired his "mental responsibility for his actions".
Mr Wragg, 38, denies murdering Jacob at home in Worthing, West Sussex.
Andrew Wragg told police his son's death had been a mercy killing
The former SAS soldier admits manslaughter on the grounds of diminished responsibility, claiming he took his son's life in a mercy killing on 24 July last year.
Jacob had Hunter syndrome, and was deaf and almost dumb.
Speaking for the defence, Dr Wright said he believed Mr Wragg had lost his mind and "actually cracked" when he claimed he had seen in his son's eyes that he did not want to go on.
The expert in combat stress added that it was clear that Jacob could not have told his father that he wished to die with a look in his eyes.
He said: "Whether it was delusional or something else, it was highly abnormal.
"We have heard again and again that it was something Mr Wragg repeated, that he had seen something in Jacob's eyes. It is highly significant.
"The process was that the combat stress from Iraq made his nervous system highly stressed. His high tension became depression.
"Seeing Jacob was the final thing that made him snap."
He went on to tell the court that while in Iraq, Mr Wragg had seen two friends shot and an Iraqi driver killed.
The trial continues.