Dr Nigel Eastman, a forensic psychiatrist, said Jamie Petrolini, 19, had a personality 'vulnerable to disintegration' and had been out of touch with reality with he slashed the throat of Mohammed el-Sayed after jumping into his car at a junction in Bayswater, west London, on 14 January.
Petrolini and Richard Elsey, 19, a fellow A-level student at an Oxford crammer, deny murdering Mr el-Sayed after selecting him at random. Petrolini has admitted manslaughter on the grounds of diminished responsibility, claiming he believed Elsey was an SAS officer and the killing was his initiation into the regiment.
Yesterday, Mr Eastman said that there was nothing to suggest Petrolini was naturally violent, nor did his mental symptoms suggest violence and the event was unlikely to have occured without Elsey.
Petrolini's mother, Vanda, told of how she saw her son after he was charged: 'He went like a rod. He said 'I did it for Queen and Country', it was a mission and that the SAS would get him out of prison.'
Elsey, of Beaconsfield, Buckinghamshire, claims he was not in the car when Mr el-Sayed was stabbed and that he believed Petrolini was going to rob not kill the man.
Dr Eastman said that Petrolini's sense of identity was so lacking he was 'vacuous'. He believed the early signs of mental abnormality could be traced to his schooldays at Gordonstoun, where Petrolini claims he felt isolated, alone and cried most nights.
The case continues.