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Several real-world mental disorders are associated with a lack of empathy. The real-world situation informs fiction but neither really mirrors the other.
Autistic people and those with Aspergers Syndrome are sometimes held to lack empathy, but it's contended that that they are just bad at expressing empathy. It is also harder for autistic people to tell what someone else is feeling - a problem that easily tips over into a perception of a lack of empathy. In other words, it's not that they don't feel empathy so much as they're bad at perceiving its need and getting others to perceive it in them. This unfortunately contributes to the image of Asperger's Syndrome as being used by people over the Internet as an excuse for being a Jerkass Know-Nothing Know-It-All.
People with narcissistic personality disorder can lack empathy. The idea is that narcissists are too busy with themselves to consider other people's feelings. Related to narcissism (at least according to Wikipedia) is a condition known as schizoid personality disorder. The conditions of these are not so much lack of empathy as the inability to express feelings, but the result ends up looking the same (i.e. the person appears "not to care"). The distinction between this and narcissism is that the narcissist tries to set themselves above other people, while the schizoid is away from them, an introvert withdrawing from others (Emotionless Girl and Hikkikomori are a good example of the kind of behavior demonstrated, only they are both present).
People with borderline personality disorder are also often accused of lack of empathy but most of the time they are just reckless, irresponsible, and don't like to talk about issues (relationships or otherwise).
In general, sociopaths experience the world so differently as to be effectively unperceivable by those with any grasp of empathy. They feel anger but it tends to be short-lived, often lasting only seconds, though the anger is often explosive and uncontrolled. They are very egocentric and callous, and they do not appreciate the consequences of their actions. They can be, however, very, very good at reading other peoples emotions and imitating them; this enables them to be highly sociable and some people live normally with undiagnosed ASPD for years and by extension all their lives, appearing to be a law-abiding and healthy member of society simply by mimicking the law-abiding behaviour of those around them (although they do not grasp why this is the accepted social standard).
ASPD does not necessarily mean the sufferer has high intelligence, but this assumption was probably gained from their Manipulative Bastard abilities. It is considered almost untreatable by those in the psychiatric profession as many of them see their doctor as just another person to be manipulated.
While sociopaths have a degree of overlap with narcissists and the like, the difference is that the Sociopath won't take it personally if you humiliate him and kick him out. The only reason he'll stay in somebody's life is because that person is gullible (or he classes that person as gullible) and there's no need to reinvent the wheel. For example, while narcissists usually see themselves above other people, when in doubt they want those very "lowly others" to take care of their needs and wants. Sociopaths will simply take what they need or want. Narcissists are still bound by emotional, moral and conscientious weaknesses, such as love or guilt, and when cornered, are far more easy to be punished. Sociopaths can easily disregard such weaknesses on the go, and simple punishment usually does not work on them.
A lack of empathy is not uncommon in serial killers, who are commonly diagnosed with ASPD. That being said, your average ASPD sufferer is more than likely to be a petty criminal, or simply a law abiding Jerkass. Also, it is important to note that lack of empathy does not make you a serial killer, or even a Jerkass, they simply make the person more noticeable in advance, if they should happen do act out in some way. It does make it more likely that the person in question will do something mean or exceedingly callous (in other words, act like a Jerkass), but this isn't automatic; it's just a consequence of failing to consider other people's feelings.