Psychologists have long-recognised the overlap that exists between aspects of Narcissistic Personality Disorder (NPD), and Psychopathy. Both are conditions that plague society in different ways. It is generally thought that these two conditions affect around one per cent of the general population; however, this changes markedly depending on which aspect of society you are looking at. For instance, a disproportionate number of the violent criminal population are psychopaths; likewise, a disproportionate number of adoptees are narcissists. Similarly, some professions seem to attract a disproportionate number of narcissists or psychopaths.
In general, narcissism can be called the "less serious" of the two, for the simple reason that the evidence seems to show how psychopaths are criminally far more dangerous (and criminally-prolific) than narcissists. Psychopathy as a syndrome is thought to be some combination of narcissistic traits and "anti-social" traits (a fuller description of psychopathic behaviour and its causes can be see here). Narcissism can be summarised as a dysfunctional self-centredness and over-evaluation of one's attributes, which uses society and other people to gain "narcissistic supply" i.e. a feeling of love and attention. We'll go into this in more detail shortly. Psychopaths share these narcissistic aspects to a greater or lesser degree, but more importantly, also have an "anti-social" personality i.e. they have no respect for society and social norms. It is this "anti-social" aspect that explains why psychopaths are, by definition, more dangerous to others and society in general. Narcissists can also be highly damaging to others around them, but is usually manifested in a different form.
One of key differences between narcissism and psychopathy is motivation. As said before, narcissists' motivation centres on finding sources of "narcissistic supply". A psychopath's motivation is more simply amoral convenience. Understanding this difference in motivation is key to understanding the differences between how narcissists and psychopaths think.
An interesting example of this is to compare the lives and motivations of two infamous serial killers. While these are "extreme" cases, the comparison in their motivations provides an intriguing insight. The serial killer, Ian Brady, killed a number of children in the 1960s in the Manchester area. There was no obvious motive for the crimes, and he said he carried out these crimes as an "existential experiment" - in other words, he did it to see what it would feel like. Once caught, he showed no remorse for the killings, and has lived his life out in comparative comfort (and living infamy) behind bars ever since. His thinking shows a complete disregard for social norms, or the acknowledgement of the seriousness of these acts. In this way, he is a fairly clear case of a psychopathic serial killer - although, some elements of narcissism were also there (as with all psychopaths). Furthermore, this psychopathic mentality displays the indifference that Brady had towards his acts: he felt like doing it, so he did it. He was indifferent to the consequences, to the victims as well as the consequences to his own life. In general, the coldness of Brady's personality (the "dead-eyed stare", a common characteristic of the psychopath) clearly demonstrated his inherent psychopathy.
This "indifference" (or "indifferent aspect") is common to psychopaths, but less so in narcissists. In the case of "psychopathic narcissists" or narcissists who take their motivation to the ultimate extreme (i.e. murder), the motivation here is more likely about "feeling like God", or a form of ultimate control over the victim. They seek "narcissistic supply" from successfully committing the ultimate social taboo, and getting away with it. However, equally, their narcissism is often ultimately their undoing, as they seek to "over-reach" (and may inwardly seek the public recognition they crave, that can only be gained through capture). A serial killer who was diagnosed has having Narcissistic Personality Disorder, Jack Unterweger, was an Austrian responsible for killing up to a dozen women, mostly prostitutes. An outgoing, larger-than-life minor "celebrity" at the time due to his published work on life in prison, his life was lived in the spotlight, first in infamy, then redemption, then fame and adulation, and finally once again in infamy. Narcissism was the driving force in his life, which ultimately led to his suicide once he had been sentenced to return to prison.
Similarities versus differences
Narcissists and psychopaths share the same perspective on society: they see society as something to be used, although they use it for different reasons. Narcissists and psychopaths use people as a leech does a host. They see people as objects. They are self-centred and lacking in empathy for others' feelings and point of view. They are entitled, feeling the right to special treatment, regardless of their lack of deserving it. Likewise, both narcissists and psychopaths are lazy in achieving goals, unable to commit to long-term plans, and quickly become bored. This attitude also overlaps into relationships and sex: they see partners as tools for their own self-enjoyment. They change their partner as soon as things start to get too "serious" or when their short attention span seeks another "conquest". Both narcissists and psychopaths have an essentially superficial view of the world.
However, there are also important differences. While both narcissists and psychopaths lack empathy and are both inherently self-centred, their behavioural aspect still bears striking differences. Lacking empathy and thus proper, in-depth emotions, both psychopaths and narcissists can also be susceptible to bursts of violent emotion. This has also been called "controlled emotion" i.e. with the appearance of a tantrum for effect.
The difference is that narcissists are more likely to show these traits more exuberantly, being prone to bouts of histrionics (interestingly, this can also be seen in homosexuals). Likewise, narcissists are more likely to outwardly display mood swings, both positive and negative, whereas psychopaths are much more likely to have the appearance of being emotionally dead. This emotional instability which is much more prevalent in narcissists stems from an inherent insecurity, which in turn stems from the root of the individual's narcissism: a traumatic or unloving childhood. Narcissism is thought to be a by-product of a lack of attention or emotionally-stunted early childhood, resulting in the child relying on its own self as a source of attention and love. For this reason, narcissists, as well as being emotionally unstable and insecure, are also likely to turn to alcohol and drug abuse, and may even - in the last resort - turn to suicide. These are all forms of "attention seeking", although to an outsider they may appear to be something quite different. At their heart, narcissists are insecure, child-like, almost pathetic, individuals, who have never truly adapted to adulthood. Being a narcissist is a fundamentally unhappy experience, where the individual is rarely - and only only fleetingly - "happy", always in search of the next source of narcissistic supply.
By contrast, psychopaths, are rarely "insecure": on the contrary, they usually have high levels of self-esteem, and are well in control of their "emotions" (if they have any); as said before, these are usually used as another tool as part of the psychopaths repertoire. Unlike the child-like narcissist, the psychopath is a true predator: lacking empathy and seeing human emotions as "weakness"; they can also be extremely adept at knowing how to manipulate others. This is what makes psychopaths so dangerous. Narcissists, being more self-obsessed, are (arguably) less so. Psychopaths are thought to be a by-product of a combination of biological and environmental factors, and thus for this reason, a different type of beast from a narcissist.
While the narcissist can be symbolised as an annoying, needy, infantile child, the psychopath is the "wolf in sheep's clothing", always on the prowl, while acting as the innocuous "Samaritan". In this sense, narcissists are also often much more easy to spot due to their overblown personalities; the psychopath is more skilled at hiding their true nature, making him even more dangerous.