The big question is, of course, how can you know whether someone is a sociopath or not? It is a difficult question and even experts on the subject can be fooled. If you suspect that someone close to you is a sociopath, I suggest you read both of the books I mentioned and think hard about it. Compare that person to the other people in your life. Ask yourself these questions:
1. Do you often feel used by the person?
2. Have you often felt that he (or she) doesn't care about you?
3. Does he lie and deceive you?
4. Does he tend to make contradictory statements?
5. Does he tend to take from you and not give back much?
6. Does he often appeal to pity? Does he seem to try to make you feel sorry for him?
7. Does he try to make you feel guilty?
8. Do you sometimes feel he is taking advantage of your good nature?
9. Does he seem easily bored and need constant stimulation?
10. Does he use a lot of flattery? Does he interact with you in a way that makes you feel flattered even if he says nothing overtly complimentary?
11. Does he make you feel worried? Does he do it obviously or more cleverly and sneakily?
12. Does he give you the impression you owe him?
13. Does he chronically fail to take responsibility for harming others? Does he blame everyone and everything but himself?
And does he do these things far more than the other people in your life? If you answered "yes" to many of these, you may be dealing with a sociopath. For sure you're dealing with someone who isn't good for you, whatever you want to call him.
I like Martha Stout's way of detecting sociopaths: "If ... you find yourself often pitying someone who consistently hurts you or other people, and who actively campaigns for your sympathy, the chances are close to one hundred percent that you are dealing with a sociopath."
WHAT DO THEY WANT?
This is an interesting question. Of course most of our purposes are strongly influenced by our connections and affections with others. Our relationships with others, and our love for them, give us most of the meaning in life. So if a sociopath doesn't have these things, what is left? What kind of purposes do they have?
The answer is chilling: They want to win. Take away love and relationships and all you have left is winning the game, whatever the game is. If they are in business, it is becoming rich and defeating competitors. If it is sibling rivalry, it is defeating the sibling. If it is a contest, the goal is to dominate. If a sociopath is the envious sort, winning would be making the other lose or fail or be frustrated or embarrassed.
A sociopath's goal is to win. And he (or she) is willing to do anything at all to win.
Sociopaths have nothing else to think about, so they can be very clever and conniving. Sociopaths are not busy being concerned with relationships or moral dilemmas or conflicting feelings, so they have much more time to think about clever ways to gain your trust and stab you in the back, and how do it without anyone knowing what's happening.
One of the questions in the list above was about boredom. This is a real problem for sociopaths and they seem fanatically driven to prevent boredom. The reason it looms so large for them (and seems so strange to us) is that our relationships with people occupy a good amount of our time and attention and interest us intensely. Take that away and all you have is "playing to win" which is rather shallow and empty in comparison. So boredom is a constant problem for sociopaths and they have an incessant urge to keep up a level of stimulation, even negative stimulation (drama, worry, upset, etc.).
And here I might mention that the research shows sociopaths don't feel emotions the same way normal people do. For example, they don't experience fear as unpleasant. This goes a long way to explaining the inexplicable behavior you'll see in sociopaths. Some feelings that you and I might find intolerable might not bother them at all.