Professor Simon Wessely, of King’s College London, said that it was a misconception that horrific crime perpetrators like Anders Brevik were suffering with mental health problems to have committed such heinous of acts.
It was a simple response to mass killings that those who committed such crimes ‘must be mad’.
But he said the way Breivik carried out the killings suggested otherwise.
A leading psychiatrist, Prof Wessely said the idea a psychiatric diagnosis could help people avoid punishment was also wrong.
Writing in the Lancet medical journal, Professor Wessely said putting forward a mental illness defence in the UK could lead a person spending more time behind bars than otherwise.
The forensic psychiatry system was not a soft or popular option, he added.
The psychiatrist also said the Breivik case highlighted another misconception - that outrageous crimes must mean mental illness.
Breivik who has been said to be suffering from schizophrenia by psychiatrist in the trial would have to be seen in the context whether his actions resulted due to delusions the professor said.
But he added the meticulous way in which he had planned and executed his attacks did not suggest that he was disorganised of schizophrenia.
Breivik who is currently facing trial in Norway has admitted to killing 77 people in Oslo and on Utoeya island last July but denies criminal responsibility.
Two reports have been compiled on his mental state and have come to opposing views on his sanity.
The court's ruling on this is to determine whether Breivik is to be sent to jail or into psychiatric care.
Breivik had argued that he was not insane and should either be put to death or acquitted.
He had blasted his psychiatric report as lies to a tabloid he had written that 80% of the report was wrong.
Psychiatrists Torgeir Husby and Synne Soerheim had concluded that he suffered from paranoid schizophrenia, but some experts have questioned the diagnosis.
The psychiatric assessment diagnosing him as a paranoid schizophrenic was controversial in Norway and the court has ordered a second evaluation to be published on 10 April.