We all heard the terms "psychopath" or "sociopath". These are the old names for a patient with the Antisocial Personality Disorder (AsPD). It is hard to distinguish narcissists from psychopaths. The latter may simply be a less inhibited and less grandiose form of the former. Indeed, the DSM V Committee is considering to abolish this distinction altogether.
Still, there are some important nuances setting the two disorders apart:

As opposed to most narcissists, psychopaths are either unable or unwilling to control their impulses or to delay gratification. They use their rage to control people and manipulate them into submission.

Psychopaths, like narcissists, lack empathy but many of them are also sadistic: they take pleasure in inflicting pain on their victims or in deceiving them. They even find it funny!

Psychopaths are far less able to form interpersonal relationships, even the twisted and tragic relationships that are the staple of the narcissist.

Both the psychopath and the narcissist disregard society, its conventions, social cues and social treaties. But the psychopath carries this disdain to the extreme and is likely to be a scheming, calculated, ruthless, and callous career criminal. Psychopaths are deliberately and gleefully evil while narcissists are absent-mindedly and incidentally evil.
From my book "Malignant Self Love - Narcissism Revisited":

"As opposed to what Scott Peck says, narcissists are not evil – they lack the intention to cause harm (mens rea). As Millon notes, certain narcissists 'incorporate moral values into their exaggerated sense of superiority. Here, moral laxity is seen (by the narcissist) as evidence of inferiority, and it is those who are unable to remain morally pure who are looked upon with contempt.' (Millon, Th., Davis, R. - Personality Disorders in Modern Life - John Wiley and Sons, 2000). Narcissists are simply indifferent, callous and careless in their conduct and in their treatment of others. Their abusive conduct is off-handed and absent-minded, not calculated and premeditated like the psychopath's."

Psychopaths really do not need other people while narcissists are addicted to narcissistic supply (the admiration, attention, and envy of others).

Millon and Davis (supra) add (p. 299-300):

"When the egocentricity, lack of empathy, and sense of superiority of the narcissist cross-fertilize with the impulsivity, deceitfulness, and criminal tendencies of the antisocial, the result is a psychopath, an individual who seeks the gratification of selfish impulses through any means without empathy or remorse."