Sex predator Robert Howard was a psychopath who posed a severe risk to schoolgirl Arlene Arkinson, according to a top psychiatrist who examined him in prison.
Dr Ian Bownes, a consultant forensic psychiatrist, also told an inquest the cunning child killer honed his manipulation skills over decades and targeted victims using a sophisticated selection process.
He said: "It is highly probable that Mr Howard would have had only one interest in Miss Arkinson and that would have been in fulfilling his sexual desires."
Fifteen-year-old Arlene, from Castlederg in Co Tyrone, vanished after a night out across the border in Co Donegal.
She was last seen being driven off during the early hours of August 14 1994 down a country road with Howard.
Her body has never been found.
Dr Bownes said Howard, who died in prison last year, would have interpreted the teenager's presence in his car as consent and he would have been enraged at any attempt to resist him.
"His whole activities would have been trained towards obtaining his sexual interests within a short space of time of being in contact with her," he said.
"Any sort of affirmation, a smile or something innocuous would be taken as full consent for some sort of sexual activity."
Howard was acquitted of Arlene's murder in 2005 by a jury which, for legal reasons, was not told of his lengthy criminal history, which included two jail terms for rape and the killing of south London teenager Hannah Williams in 2001.
A violent physical attack could have precluded a sexual assault on Arlene, the court heard.
Dr Bownes said: "It is highly probably if he had expended quite a lot of money and effort towards winning Arlene over, if she had in any way resisted he would have got annoyed and felt thwarted in his sexual activities."
Howard was interviewed by the psychiatrist on two occasions in November 1994 while he was being held on suspicion of raping another teenage girl in Castlederg.
He had tried to manipulate test scores and repeatedly contradicted himself, it was revealed.
In a report which was read to the court, Dr Bones said: "His primary aim was not to provide me with information but to answer the questions in a way that would present him in the most favourable light.
"I feel Mr Howard tended to omit those questions to which it was unclear what the right response would be.
"He had an ability to behave in a calculated manner and there was an unwillingness to present information which would paint him in an unfavourable light."
Howard also tried to explain his preference for teenage girls.
He told the doctor: "Maybe I cannot accept I am getting older; I prefer younger people's company in general; it could be a fear of dying and; it is less work to get a young girl into bed than a 50-year-old woman."
His sexual fantasies included forcing someone to do something, it emerged.
The doctor concluded Howard had probably used his girlfriend, identified during other hearings as Patricia Quinn, to conceal his activities from the local community.
"He needed a lair from which to branch out and target his victims and what better than to have a relationship with someone."
Rehabilitation at the age of 51 was also unlikely, he added.
"Mr Howard had been a psychopath for many years at the time when I examined him so the chances of changing his behaviour would have been slim."
Meanwhile, coroner Brian Sherrard has said he plans to write to the Commissioner of An Garda Siochana, Noirin O'Sullivan to request co-operation on the inquest.
He said: "My proposal is to send a letter to the Commissioner to see if they can offer any assistance.
"There are a number of discreet matters that the Guards can assist us with."
The case has been adjourned until Wednesday.
