The most interesting part is how unlike a typical disorder it is. Your typical disorder removes capability; psychopathy removes empathy, which can be seen as a capability- but it could also be seen as a natural (well-intentioned) limiter.
I don't think it is evolutionary stable. I think a society of sociopaths would fall apart. We evolved to live in groups; we are social animals. Social animals need empathy, otherwise they are unlikely to cooperate and live in mutually beneficial ways.
"Psychopath" could just be the degenerate form of the personality type that evolved to lead those groups. In other words, a leader is a psychopath with better impulse control.
researchers could study only the psychopaths’ behavior, in most cases through their criminal records
It's a fascinating disorder, really, in that the most severe cases are difficult to detect so long as they do not make mistakes.