A teenager's claim that severe hallucinations compelled him to kill two strangers has been described as "cliched" and "unconvincing".
Psychiatrist Dr Philip Joseph, who assessed the teen earlier this year, told his trial his defence sounded "more like something you might see in a horror film".
The 17-year-old, who cannot be named for legal reasons, was just 15 when he killed strangers James Attfield, 33, and Nahid Almanea, 31, in Colchester in 2014.
He has admitted manslaughter on the grounds of diminished responsibility, but denies murder.
Dr Joseph said the boy had described severe auditory and visual hallucinations, including one incident where a hand and arm emerged from inside a television set.
But he said it was "inconceivable" that no one would have noticed if the teen had been suffering from hallucinations, which he had said were so loud at times that he thought others might be able to hear them.
"If he was experiencing these voices to such an intensity over such a prolonged period, is it really feasible that nobody would have known that he was having a problem of this sort?"
He added that the voices were more likely have been a "fallback option" to tell police when he was arrested.
Dr Joseph instead concluded the teen could be suffering from an emerging personality disorder which makes people callous, devious and manipulative.
Mr Attfield was knifed 102 times, on 29 March, 2014.
Ms Almanea, a Saudi student, was stabbed to death along the Salary Brook Trail on the morning of 17 June 2014.
The boy gave a graphic description of the killings when he was arrested in May last year, but later claimed not to have been responsible, saying the voices told him to make an admission.
He now says he still cannot remember the incidents but says he must have committed them due to the presence of DNA evidence.