Sociopaths have no empathy for other people. That’s what allows them to do the things in @Darwin‘s list. A note: sociopath and psychopath seem to be synonymous terms.
From Suffering Souls
Psychopaths don’t exhibit the manias, hysterias, and neuroses that are present in other types of mental illness. Their main defect, what psychologists call “severe emotional detachment”—a total lack of empathy and remorse—is concealed, and harder to describe than the symptoms of schizophrenia or bipolar disorder. This absence of easily readable signs has led to debate among mental-health practitioners about what qualifies as psychopathy and how to diagnose it. Psychopathy isn’t identified as a disorder in the Diagnostic and Statistical Manual of Mental Disorders, the American Psychiatric Association’s canon; instead, a more general term, “antisocial personality disorder,” known as A.P.D., covers the condition.
At least one researcher, Dr. Kent Keihl, believes that psychopathy has an organic root cause:
In January of 2007, Kiehl arranged to have a portable functional magnetic-resonance-imaging scanner brought into Western—the first fMRI ever installed in a prison. So far, he has recruited hundreds of volunteers from among the inmates. The data from these scans, Kiehl hopes, will confirm his theory, published in Psychiatry Research, in 2006, that psychopathy is caused by a defect in what he calls “the paralimbic system,” a network of brain regions, stretching from the orbital frontal cortex to the posterior cingulate cortex, that are involved in processing emotion, inhibition, and attentional control. His dream is to confound the received wisdom by helping to discover a treatment for psychopathy. “If you could target the brain region involved, then maybe you could find a drug that treats that region,” he told me. “If you could treat just five per cent of them, that would be a Nobel Prize right there.” (from the article cited above).
So, to conclude, sociopathy is a condition that psychiatrists call “antisocial personality disorder” and it is difficult diagnosis it. Some time in the future, if the fMRI study turns up a correlation, it may be possible to diagnose it with brain scans. It may also be possible to treat it with medication.