As I entered the lift of my hotel in Wichita, Kansas, two other people joined me. Both were dressed as animals.

“Which floor?” asked the largest who was dressed as, I think, a biker mouse.

“8 please,” I replied, and we then stood in total silence waiting for the doors to close.

The biker mouse’s companion seemed to be a stoat, although it was hard to tell given that most of his fur was acid green. All I knew was that no trace of human being pierced either of their costumes.

Later I’d discover that there was a “furry” convention booked into my hotel and so my days in Wichita would start and end with about a hundred people dressed as animals, or more exotically as pseudo-animals, milling around drinking coffee in the morning, or beer at night. Even at the time, this seemed like a weirdly perfect motif for the blurred line between normality and oddness that had brought me to Kansas, and one which resonated with the man that I had come to study.

Wichita was the home and killing location of Dennis Rader, a serial killer who dubbed himself “BTK” – bind, torture and kill - and who, for a number of reasons, is one of the stars of that deadly genre.

My interest in Rader goes back a long way, partly because he is almost the complete embodiment of both the serial killer and the psychopath. A church-going, scout-leading, family man, he also had a lifelong interest in bondage and sadism – he would call his penis “Sparky” in his writings that were later discovered after his arrest - which he indiscriminately inflicted on the ten people he murdered between 1974 and 1991, although he wasn’t actually arrested until 2005.

Like my furry friends in the lift, Rader too was disguised, only in his case it was as a human. It was a disguise that he had almost perfected, even helping the police after he had been employed as a local compliance officer.

Little about Rader fits the “typical” pattern of a serial killer. Most serial killers, for example, start small; they will have had a history of violent fantasies which will only very slowly get turned into reality. So they usually begin by assaulting people and then, bit by bit, as their fantasies take over and become more demanding, they start to kill.

Not Rader. His first victims were an entire family – the Oteros - whom he killed in January 1974.

Even during this first murder of Joseph Otero (38), his wife Julie (34) and their two children Josephine, aged eleven and her seven-year-old brother Joey, he was in almost total control of the crime scene. Normally we expect the serial killer’s first kill to be somewhat botched and therefore a source of potential evidence. No such luck with the Oteros, where Rader showed ingenuity and complete control. For example, he secured Joseph and Julie’s compliance and they allowed him to tie them up, promising them that he just wanted their car and some money and then he would be on his way. Perhaps Joseph and Julie thought that if they didn’t resist Rader might leave more quickly, or at least not harm their two youngest children.

‘Geographically stable’, as opposed to ‘transient’ serial killers, are usually caught quite quickly but, despite the fact that Rader killed entirely within Wichita and its environs, he escaped justice for over 30 years. He also took long breaks between his murders so that, for example, there was almost eight years between his murder of Nancy Fox in December 1977, which he described as his “perfect hit” and Marine Hedge – his next victim - in April 1985.

This too is unusual for serial killers - they don’t usually switch on and then off again their desire to kill. Killing is a compulsion for them, propelling them ever onwards to more victims and often in increasingly bizarre circumstances. This might make it appear that they want to be caught. Nothing can be further from the truth although, by the time that justice finally catches up with them, they have become so divorced from reality that they often simply don’t realise how strange their behaviour has become. They therefore take risks and impulsively throw their plans – their modus operandi, their criminological “signature” out of the window because they believe that this is no longer going to deliver a kill.

Dennis Rader is escorted into the El Dorado Correctional Facility on August 19, 2005 in El Dorado, Kansas (Getty Images)

Rader, like some serial killers, regularly communicated with the police and also with a local newspaper called The Wichita Eagle but, atypically, it was he himself who coined the name “BTK”. Communication in this way allows the killer to feel powerful; to be in charge; to be dominant.

But then the communication stopped, reflecting a degree of self-control, rather than the impulsive fantasies of his kills. Wichita secretly hoped that the BTK had gone away – that he had died, or been incarcerated for other crimes.

It was only in 2004 that Rader started to get back in touch with the police – some thirteen years after his final murder of Dolores Davis (62) in January 1991. He was prompted to do so only because he was worried that in his 30 anniversary killing year, others were being given credit for the BTK murders and so Rader was less than pleased. It was a mistake because the police were soon able to identify the location of the computer that he was using to write to them from – the one in the Lutheran church where Rader was President.

Filming in Wichita, driving to all the key locations and deposition sites, and talking with the police, court staff and the journalists who worked the case, and with Rader’s former neighbour, I could see that he was almost the “perfect” psychopath. Think of psychopathy as a personality disorder defined by a cluster of traits centred around three different factors which, over time, have become ingrained as beliefs and behaviours.

First, is their inter-personal style, which allows the psychopath to be glib, grandiose, dishonest and manipulative; they are always arrogant and deceitful in their day-to-day dealings. Second, as far as their behaviour is concerned, psychopaths will be sensation seeking, impulsive, reckless to the point of stupidity - seemingly having no thought for their own safety. Finally, psychopaths will have defective emotional responses so that they lack remorse for their manipulative, reckless behaviours and find it impossible to truly understand why it is that you might actually find their behaviour wrong. In short, they just don’t get it; they operate in a totally different moral universe.

Rader could often be reckless in how he went about his murders – despite planning them with care; manipulative of his immediate family and of his community; arrogant in his demand for attention; and grandiose in his belief that he would not be caught and that he had somehow befriended the police officer assigned to catch him. He gave a jaw-droppingly insensitive performance in front of the judge and the families of his victims. He was at last publicly living the life less ordinary that he had always craved, and could only previously achieve in private when he killed.

Yet, according to his son, Rader was also a “perfect father” and Paula Rader described her husband to the police as “a good man, a great father. He would never hurt anyone.” Indeed, that was the face that Rader liked to present to the community – to the Scouts in his troop and to the congregation of the church that he attended. Psychopaths, like Rader, often hide in plain sight and, as I have often explained, if they had horns on their heads and long, pointed tails, psychopaths would be much easier to identify and therefore to avoid.

However some disguises are just too difficult to spot, until it’s too late.