Psychopathy is a congenital inability to feel empathy, remorse, or any of the emotions necessary to form a conscience, coupled with a high need for stimulation, impulsiveness, opportunism, often culminating in a callousness disregard for the rights and feelings of others. Such people become tend to become predatory toward their fellow human being, and when they do, they closely resemble what most people would regard as “true evil.”
There are some studies of twins reared apart that strongly suggest that psychopathy may be inherited. Robert Hare (and others) have gone so far as to described psychopathy as a phenotype—a distinctive kind of trait arising through the interaction between the genome and the environment—as opposed to a developmental condition. In this respect, psychopathy appears linked to autism, which also involves a lack of empathy, little ability to form friendships, one-sided conversation, and difficulties in handling external stimulation.
I agree with @dpworkin, however, that the genetics involved are anything but straight-forward, especially since both conditions exist in varying degrees; so it is not a simple “on-off” affair. It very likely involves a complex of interacting genes, and perhaps even metagenetic factors. The search is on for brain imaging tests that can reliably detect who is and who is not a psychopath. The researchers know that there is definitely faulty wiring in the brains of psychopaths, but there is a lot of faulty wiring in non-psychopath’s brains too; so the difficulty is in devising a test with enough specificity that it does not return unacceptably high levels of false positives while still detecting the characteristic of interest.
Now to the issue of whether such a technology should be used once it is perfected: Certainly it would make sense to screen people out of sensitive positions so that they don’t become political leaders. The temptation, however, would be to remove them from all walks of life, or at least put a tag on them so that they can be sequestered if they become truly dangerous. That sets up some ominous precedents if they haven’t committed any crime, or if they are thrown into prison where they are made orders of magnitude worse. Psychopaths are notoriously resistant to any conventional form of treatment—in fact, it actually seems to make them worse. They seem to do well in armies because they can kill without any stress or remorse.
From an evolutionary biology point of view, predators greatly accelerate the evolution of a species, so there may be hidden benefits to the ostensible costs of all the criminality.