To grossly oversimplify it, narcissists aren't as bad as sociopaths.
Compare the diagnostic criteria for Antisocial Personality Disorder and Narcissistic Personality Disorder. Here's a link to the current DSM-5 diagnostic criteria for personality disorders. As far as I know, they haven't changed much from the DSM-IV.
http://www.psi.uba.ar/academica/carrerasdegrado/psicologia/sitios_catedras/practicas_profesionales/610_clinica_cuadrosfront_psicosis/material/dsm.pdf
Next, take a peek at the Hare Psychopathy Checklist Revised. This is a diagnostic tool for psychopathy. Sociopaths and psychopaths score over 30. As a diagnosed narcissist, I score 18. The average person will score under 10.
One of the simplest differences between ASPD and NPD is that antisocial behavior is simply for the thrill, gratification, and satisfaction of it. Because you can. It's a pattern of disdain and even hatred for other human beings, displayed by a pattern of behavior beginning in early childhood of rule breaking, illegal activity, fire setting, abuse or torture of animals, and a reckless disregard for the rights, boundaries, feelings, and safety of yourself and others. Narcissism is characterized by a similar self-centeredness, but for different reasons, and with different motivation. It's ego boosting, it's getting one's self esteem vicariously through others, it's NEEDING the approval and praise of other people.
A sociopath does not need other people. People are tools, pawns, a source of amusement, toys. They don't seek approval, praise, or acceptance. They're a whole different level of selfish, egotistical, and self-serving. Sociopaths exploit simply because they can. Narcissists exploit to gain the approval and praise of others to boost their self esteem.
Sociopaths and psychopaths suffer from what is called "flat effect". They have no emotions. What they display are brief flashes of very shallow, often entirely feigned emotions. The only one they seem to feel with any intensity is anger. They don't know guilt, remorse, shame, sadness, grief, anything. They have the emotional capacity of a goldfish. They cannot empathize with others beyond the most basic, intellectual kind. They know something is in pain by the way it squirms, but are physically incapable of relating to other creatures. Most (if not all) seem to lack the human empathy gene and medical tests suggest a dysfunction of the amygdala.
Narcissists typically can feel human emotion, often the full range, and often including guilt, remorse, and shame. While they have stunted empathy, most do nonetheless retain a capacity for empathy and often can genuinely empathize with and feel sympathy for other human beings and animals. It's just that their needs come first. Always. Often at some cost to others.
ASPD, Sociopaths, and Psychopaths as a RULE have a history of run-ins with the law and/or engaging in acts that are grounds for arrest. Narcissists typically don't. Narcissists are just a jumble of maladaptive coping mechanisms and unresolved issues, bumbling along trying to feel good at the expense of others. Sociopaths do not even comprehend the need for external validation.
Both use and, often, abuse people for personal gain, but for entirely different reasons. The biggest difference in the two is the level of deliberate, intentional, premeditated malice in their actions and the underlying origins of their disorder.
[Add] Genuine narcissism comes in varying degrees and several different flavors. Sociopathy/psychopathy can come in different varieties, but the severity is pretty constant.