Psychopaths are remorseless, lacking empathy, and without conscience. Sociopaths are the same. Narcissists are bordering on psychopathy,but do have the capacity for remorse (even if only temporary and self-justifying) as a result of having some conscience (although it is not deeply embedded in the same way as it is in a fully functioning personality).
Basically, personality disorders are on a spectrum. Within each disorder, there is variation. So, for example, one psychopath might become a serial killer, while another might become a politician.
If you're interested in this kind of thing, read the book 'Snakes in Suits'. That's a good introduction on psychopathic behavior in the workplace. There are also a ton of blogs out there for people who have survived relationships with both psychopaths and narcissists (whether as parents, lovers, or friends). Those will give you a lot of insight.
Just remember one thing - if you are coming up against someone without a conscience, don't expect to win. You need to walk away, and, if your life might be at stake, make sure you have enough evidence left behind to make sure that they are caught after your demise. Not kidding.
Personality disorders are most likely a result of early traumatic and dysfunctional child-rearing. They are considered incurable, and all are onset before the age of puberty. Medication doesn't help.