David Gilroy close-up of eyes
And they are immune to appeals to end the suffering of families because they are selfish and completely lacking in remorse.
Professor Prem Misra, one of Scotland’s most respected authorities on mental health, says the aim is not to hurt relatives of victims since the killers have paid no thought to loved ones in any case.
He said: “They are not capable of remorse or any kind of empathy with those left devastated.
Their sole concern is their own situation.
“At the back of their minds, they are aware they hold certain things in their knowledge no one else is aware of.”
Misra says this kind of behaviour can intensify once the killer is in custody or on trial and no longer has control over events.
He added: “They have been stripped of power.
“So whatever power they perceive to still be available to them becomes intensified and more important.
“This knowledge that they know something the authorities and the relatives do not know can provide a kind of inner strength.”