The killers mentioned above are all examples of the classic psychopath, defined by psychologists Hervey Cleckley in 1941 and later more precisely by Robert Hare's Pychopathy Checklist.

Cleckley published The Mask of Sanity, which listed 16 distinct clinical criteria, among them:

• hot-headed
• manipulative
• exploitative
• irresponsible
• self-centered
• shallow
• unable to bond
• lacking in empathy or anxiety
• likely to commit a wide variety of crimes
• more violent, more likely to recidivate, and less likely to respond to treatment than other offenders 

Then as the concept of psychopathy evolved, the emphasis shifted from traits to behavior, and in 1952, the word "psychopath" was officially replaced with "sociopathic personality." By 1968, "sociopathic personality" yielded to "personality disorder, antisocial type."  Yet many people working with psychopaths felt that such a label was nonspecific.

Without Conscience bookcover

Then came Robert Hare, a Canadian psychologist who had access to prison populations.  Based on Cleckley’s work, Hare combined traits and behaviors for his Psychopathy Checklist (PCL) and wrote about it in Without Conscience.  He included 22  items (twenty in the PCL-Revised), to be weighted from 0 to 2 by clinicians working with potential psychopaths. Moving away from the antisocial personality diagnosis, psychopathy was now redefined as a disorder characterized by the traits from Cleckley's list with a few more that included:  

• serial relationships (multiple marriages)
• lying
• glibness
• low frustration tolerance
• parasitic lifestyle
• persistent violation of social norms

Once they can diagnose a psychopath, clinicians know how to deal with them.  First, they understand that there is no cure for this condition and it may even worsen with psychotherapy.  Second, they know that these people are among the most dangerous criminals, having little human feeling for victims and no real concern for laws and social expectations.  People who engage in what others call evil are generally psychopaths.  Whether they're hard-wired to be what they are, taught it, made into it from brain damage or abuse, or encouraged by some faulty socialization is still the central question, and many researchers are attempting to find the answer.

Here's a brief list:

The Creation of Dangerous Violent Criminals bookcover

1.)   Dr. Lonnie Athens, author of The Creation of Dangerous Violent Criminals, takes the approach that psychopathy or antisocial behavior develops through specific steps.  He believes that people start off benign, so in an attempt to discover why only some people in a crime-vulnerable environment turn violent, he interviewed violent criminals in prisons to find out what they had in common.  From his research, he determined that people become violent through a process of "violentization," which involves four stages:

• brutalization and subjugation
• belligerency
• violent coaching
• criminal activity

First, the person (usually a child) is the victim of violence and feels powerless to avoid it.  Then he is taught how and when to become violent (often by a person who was violent to him) and how to profit from it.  It's not long before he's had sufficient exposure to act on it.  According to Athens, if someone from a violent environment does not become violent, it's because some part of the process is missing.  Athens seems not to include an inherent tendency toward violence, narcissism, or shallow emotion as part of the antisocial scenario.  It's also clear from some of our examples that his theory will not apply to all acts of evil—even of violent evil.

2)   Dr. Stanton E. Samenow, an authority on the criminal personality and a former member of Reagan’s task force on crime victims, insists that the criminal’s way of thinking is vastly different from that of responsible people, and that the “errors of logic” derive from a pattern of behavior that begins in childhood.  Criminals, he says, choose crime by rejecting society and preferring the role of a victimizer.  They are in control of their own actions, but they assign the blame for their behavior to others.  Thus, they have no insight about their intentions.  They devalue people and exploit others insofar as those others can be manipulated toward ends to which the criminals feel entitled.  The excitement of crime, they believe, staves off emptiness, but satisfaction doesn't last long, so they do it again.  They don't learn because they'd don't think correctly.

Ghosts in the Nursery bookcover

3)   According to Robin Karr-Morse and Meredith S. Wiley in Ghosts in the Nursery, the roots of violence develop in the first two years of life, starting at conception. "With the exception of certain rare head injuries," they claim, "no one biological or sociological factor by itself predisposes a child to violent behavior. The research underscores that it is the interaction of multiple factors which may set the stage." In other words, it's not due to a negative experience, a brain disorder, genetics, or mistakes in parenting, but it could be the result of the cumulative effect of a combination of factors, along with the failure of normal protective systems in the environment.

Among those factors associated with violence, they list

• harmful substances ingested by mothers during pregnancy
• chronic maternal stress during pregnancy
• low birth weight
• early maternal rejection or abuse
• nutritional deficiencies
• low verbal IQ
• ADHD

While none are considered causal, in certain combinations and with certain dispositions, they can provoke anger, lack of anger management skills, and violence against self or others. If kids fail to connect early with caregivers, there can be problems later in life. "Babies reflect back what they absorb," the authors say, and that notion has serious implications. If we fail to address the issues of competent child-rearing and healthy pregnancies, one in twenty babies born today will end up behind bars.

The Biology of Violence bookcover

4)  Debra Niehof, a neuroscientist, studied twenty years' worth of research before she wrote The Biology of Violence.  Specifically, she wanted to know whether violence is the result of genes or a product of the environment.  There are several studies that indicate that the physiology of a psychopath is somehow different, that something isn't quite right in their brains, and that they aren't as responsive to punishment.  However in Niehof's opinion, both biological and environmental factors are involved, and each modifies the other such that processing a situation toward the end of a violent resolution is unique to each individual.  In other words, a particular type of stimulation or overload in the brain is not necessarily going to cause violence in every instance.  (Other young men watching The Billionaire Boys' Club did not decide to murder their parents.)

The way it works is that the brain keeps track of our experiences through chemical codes. When we have an interaction with a new person, we approach it with a neurochemical profile, which is influenced by attitudes that we've developed about whether or not the world is safe, whether people are trustworthy, and whether we can trust our instincts.

However we feel about these things sets off certain emotional reactions and the chemistry of those feelings is translated into our responses. "Then that person reacts to us," says Niehoff, "and our emotional response to their reaction also changes brain chemistry a little bit. So after every interaction, we update our neurochemical profile of the world."

The chemistry of aggression is associated with the chemistry of our attitudes and we may turn a normally appropriate response into an inappropriate response by overreaction or by directing it to the wrong person.  In other words, the person's ability to properly evaluate the situation becomes impaired. Niehoff says that there are different patterns of violent behavior and certain physiological differences are associated with each pattern.

While most violence occurs under provocation of some type, certain people initiate it for pleasure and erotic stimulation, as we've seen with Bundy, Dahmer, and Nilsen.  Some are psychotic, some are provoked by substances, and some use violence as a weapon. There are also killers for whom violence is the only way to satisfy their lust.  They're driven by the need for this form of arousal. 

Yet even for them, the development of these behaviors results from a cumulative exchange between their experiences and the nervous system.  It all gets coded into the body's neurochemistry as a sort of emotional record.  The more they succeed and feel the high, the more likely it is that they will return to this behavior.

It seems, then, that evil is a complicated human behavior for which it's difficult to find a cause, and since we have the cognitive means for viewing evil acts from perspectives that diminish their heinousness, our capacity to commit evil increases---especially if we believe we're doing it for some noble purpose.