Welcome back to the Personality Disorder Parade. I'm starting with some of the more prominently known personality disorders (or least the ones you might hear thrown around on TV, pop culture, that sort of thing). Antisocial PD will be first.
The main feature of Antisocial PD is a pervasive pattern of disregard for, and violation of, the rights of others than begins in childhood or early adolescence and continues into adulthood. This is also called psychopathy or sociopathy.
For this diagnosis to be given to someone, they have to be over 18 and have a history of some symptoms of Conduct Disorder or a diagnosis of it. Children with Conduct Disorder repeatedly violate the basic rights of others and age-appropriate norms. It's IMPORTANT to remember the following four categories of behavior when giving your villain (or whatever character) a backstory: aggression to people or animals, destruction of property, deceitfulness of theft, or serious violation of rules.
So once they are adults, this pattern continues. They fail to conform to social norms with respect to lawful behavior. So whether of not they get arrested, they do things that would definitely land them in jail if they were caught, and they do these actions with disregard for the wishes, rights, or feelings of others.
They are frequently deceitful and manipulative in order to gain personal profit or pleasure (i.e., wanting money, sex, power, etc) and they may repeatedly lie, use an alias, con others or malinger (feign illness).
These individuals are impulsive, and fail to plan ahead. This means they make decisions without consideration for the consequences to others or to themselves...so they might have sudden job changes, residence changes or relationship changes. They are also extremely irresponsible and may be unable to hold down a job or defaulting on debts/alimony.
People who suffer from APD tend to be irritable and aggressive, repeatedly getting into fights or committing acts of physical assault. These are child beaters, spouse beaters, etc, but the aggressive act isn't in defense of themselves or someone else (so these people don't try to defend someone's honor by getting into a fight...they just pick a fight or punch someone for no reason, that type thing). They have a reckless disregard for the safety of themselves OR others. They may be speed racers on the highway, driving under the influence, engage in sexual behavior of substance abuse with high risks for harmful consequences...and they just don't care. You don't want to have someone with APD solely in charge of taking care of a child, for instance.
Lastly, these individuals show little remorse, if any, for the consequences of their acts. They may be indifferent or give some rationalization for having hurt, mistreated or stolen from someone, like "$#!t happens," or "he had it coming." They may blame the victim for being stupid or in the wrong place at the wrong time.
To give you an idea, here's a quote from Wikipedia about Ted Bundy.
"On death row, Bundy admitted to decapitating at least a dozen of his victims with a hacksaw. He kept the severed heads later found on Taylor Mountain (Rancourt, Parks, Ball, Healy) in his room or apartment for some time before finally disposing of them. He confessed to cremating Donna Manson's head in his girlfriend's fireplace. Some of the skulls of Bundy's victims were found with the front teeth broken out. Bundy also confessed to visiting his victims' bodies over and over again at the Taylor Mountain body dump site. He stated that he would lie with them for hours, applying makeup to their corpses and having sex with their decomposing bodies until putrefaction forced him to abandon the remains. Not long before his death, Bundy admitted to returning to the corpse of Georgeann Hawkins for purposes of necrophilia (sex with corpses)."
Now for some general character traits associated with Antisocial PD, but not necessary for an actual diagnosis of it (take note, you'll want to include these in your characterization):
lack of empathy, callous, cynical, inflated/arrogant self-appraisal (they are better than certain types of work or people), opinionated, self-assured, cocky; glib, superficially charming, exploit sexual relationships, or easily bored.
Those traits in bold are features that have been commonly included in the traditional concepts of psychopathy that may be particularly telling of the disorder and more predictive of convicted criminals who repeatedly offend (Jeffrey Dahmer, Ted Bundy, etc). This disorder is found in about 3% of males and 1% of females in general (higher percentages are found in substance abuse treatment settings, prisons and forensic settings). Antisocial PD is chronic, but tends to become less evident as the person grows older, particularly by the time they reach their 40s. Just FYI.
As you can see, this is the personality disorder of choice (not that they actually have a choice) for serial killers, sociopaths and psychopaths. Hopefully this rundown will give you some ideas for your diabolical stories.
Q4U: What did you learn new about this disorder? Did you know that sociopath wasn't really a "disorder" per se, and that the real associated name was Antisocial Personality Disorder?