Several real-world mental disorders are associated with a lack of empathy. The real-world situation informs fiction but neither really mirrors the other.
Antisocial Personality Disorder
Of course, we have to mention Antisocial personality disorder, more popularly known in the past as The Sociopath. 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 people's 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.
ASPD vs Serial Killers
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 Jerk Ass. Also, it is important to note that lack of empathy does not make you a serial killer, or even a Jerk Ass, 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 Jerk Ass), but this isn't automatic; it's just a consequence of failing to consider other people's feelings. Despite the criminal justice system's treatment of premeditated murder in favor of crimes of passion, it is important to note that all people without emotions are not serial killers (they might in fact more likely be schizoid, which in fact are for the most part harmless, and are more in danger of being alone their whole lives than going on a murdering spree). The average person avoids the unshaven type but a serial murderer is actually more likely to be handsome, charming, and seemingly normal.
These are other disorders that either cause or get associated with a lack of empathy Lack of empathy is one of the diagnostic criteria for Narcissistic personality disorder. People with this disorder are too busy with themselves and simply don't care about thoughts and feelings that conflict with their own. Many of them consider other people as just tools to get what they want and need, which is normally praise and attention to maintain their grandiose self-image. People with Schizoid personality disorder can also lack empathy. People with the disorder have flattened affect and therefore is hard for them to feel for others when they donít have much feelings to begin with. Adding to this, they also have an aversion and/or inability to express the feelings they do have so even when they do care they will still appear cold and indifferent. 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). Those with Histrionic personality disorder can be unempathetic, stemming from being so focused on a group of people they lose sight of the individuals making up that group. Unlike The Sociopath they are more embracing of social groups but will move on to other people, with no regard to what effect this has, if they don't find enough stimulation there.