Lack of empathy doesn’t always mean evil. Sometimes it can just mean lack of understanding…the inability to put oneself in another’s shoes.
For instance, a coworker might be at her desk, crying because her boyfriend just dumped her. I might not bear any ill feelings towards this coworker, but I can’t feel sorry for her. Her pain over being abandoned is so foreign to me that I cannot empathize. I might say a perfunctory “I’m sorry”, but not really mean it. In fact, I might feel like she’s stupid for even feeling anything. It’s just a guy. Why should she care what he thinks?
Even if the coworker were replaced by my sister, I would still not have empathy. But because I love my sister very much, I would feel sympathy because I wouldn’t want her to be in pain. A sociopath would lack both feelings.
(9/11 was really weird for me. My boss was standing right next to me, crying as the whole thing played out on the radio, and all I could do was wash my test tubes and say “Yes, it’s very horrible,” while inside I was thinking “Why is she being so hysterical?!” I couldn’t even feel moved to comfort her with a hug. I felt absolutely nothing until I got home and imagined the horror of a family member or myself leaping out of one of the towers. Personalizing it that way made me sad. But without making it all about “me” or someone close to me, I couldn’t feel anything. Don’t worry. I feel plenty of guilt.)
I’ve been diagnosed with schizoid PD, as a few in this thread may already know. It has nothing to do with any of the cluster B PDs, although I know it can be co-morbid with borderline and perhaps narcissicism PD. Schizoids tend to be sensitive individuals, but paradoxically they can also be characterized as having “lack of empathy”–a concept I don’t think most people seem to really get. I didn’t actually understand it until just recently. It doesn’t mean someone is evil, necessarily. It can sometimes just mean someone lacks imagination. 
It must be said: There are few “pure” examples of personality disorders. Even among sociopaths/anti-socials. For instance, people might assume I have no emotions–which would be a trait of a “pure” schizoid. I’ve been described as “emotionless” and “indifferent to everything”, and I do think I’m shut off from my feelings sometimes. But I do have emotions. I’m just not very expressive, and when I do get emotional, I rarely express them in public. If I won a million dollars, you better believe I’d feel happy. You’d have a hard time finding anyone who is 100% non-emotional…unless you include people who are in a catatonic stupor.
So it’s believable that a sociopath can be indifferent to the welfare of everyone. Everyone, that is, except for his beloved mother and his dog. He’d still be a sociopath, though. Just not an archetypal one.
Also, the borderline diagnosis gets tagged to “crazy” women disproportionately. But it’s hypothesized that many men also fit the criteria for BPD and are labeled with anti-social PD–a disorder disproportionately represented by men. Take of that what you will.