Evolution "decided" (via natural selection) that having 1% of the human population as psychopaths was optimal for the survival of society. It's hard to imagine why. Perhaps psychopaths don't get PTSD in war, and keep fighting while others fall away - a distinct advantage since human history is full of wars. In modern times, so-called "corporate psychopaths" like Robert Maxwell can be very successful at the ruthless art of business warfare. Whatever the reason...
Psychopaths are important because they clearly demonstrate the distribution of traits in society. They make the case that we humans are like bees, born to perform specialized roles. Evolution decided that having 1% (not 20%, not 5%) of the population allocated as psychopaths was the right number for the survival of society over time. Psychopaths, because their behavior is so distasteful and foreign, are good subjects, because we can study them objectively.
Clearly, psychopathy is no genetic accident. It involves a highly tuned set of traits, including "use of cruelty to gain empowerment, exploitative tendencies, defiance of authority, and destructive excitement seeking" as well as "low fear, stress-tolerance, toleration of unfamiliarity and danger, and high self-confidence and social assertiveness". Psychopathology is too complex to be a mistake. It's here for a reason.
Psychopaths differ from you and me by only 0.1% of their DNA. That's not enough difference to swap in and out entire swaths of DNA for psychopathic traits. This implies we must each have the psychopathic DNA inside, with the 0.1% difference in our DNA acting as a master switch, turning psychopathy traits on and off. But it's not a satisfying answer. Serial killer Jeffrey Dahmer's parents were mild-mannered. They didn't seem to have a psychopath's master switches in their DNA to pass along to their son. So another mechanism to allocate the trait to 1% of newborns may be at play.
An alternative explanation is that such traits are allocated by a random number generator (a "dice rolling" mechanism when human sperm and eggs are generated), with a 1% chance of any child (rich or poor) being allocated as a psychopath (or a leader, etc). Perhaps the random number tagging is done via an epigenetic marker on DNA.
This begs the question: What other traits has evolution allocated (by random chance) to a small subset of society? What percent of us are born followers? born leaders? born risk takers?