Mostly a human gene pool's survival and prosperity is best served by community co-operation, mutual support and self sacrifice for the good of loved ones and other community members. Sometimes though, the group as a whole has had it and the route to the survival of any of the gene pool is for one or a few to sacrifice the rest and make an all-out bid for personal survival. E.g. a terrible winter in which there is insufficient food and everyone is starving to death.
Another situation where psychopathy could be useful is warfare. Sometimes having a cold blooded killer with no remorse on your side isn't such a bad thing.
Ultimately, it's a matter of variety. You never know what challenges your community is going to face, and what range of behaviors are going to be optimal. Having a full range of behavior types available in your community gives it flexibility in it's response, especially to to existential challenges.
[Edit: Afaik, psychopathy is not turned on after someone reaches adulthood. An evolved strategy for catastrophes would be turned on at e.g. hunger or stress.]
I especially wonder if warfare is a good example? Most of the time when humans evolved we were in a clan society. Clan warriors aren't generally known for live and let live-attitudes to people from outside their clan anyway; a psychopath would probably not stand out much in blood thirst. (And if you are in a war party, you really really need to trust the members in the party. Manipulative people would not last long when they were caught at it.)
Even among tribal societies, warfare is highly ritualized in a manner that does not optimize for maximum lethality. Richard Gabriel, in studies on tribal societies in New Guinea have noted that hunts occurred with accurate feathered. Tellingly, tribal warfare employed featherless arrows. Similarly "counting coup" among American Indians involves touching rather than killing the enemy.
Cold blooded killers have their uses for society, especially in warfare. An excellent book on this subject is On Killing.
Groups of people have always been more ruthless than individuals. Psychopaths are remarkable not in their ruthlessness but in their ability to achieve it all by themselves.
It's different than the normal HN fare, but worth a read. We may be a society of killer apes, but we're not as eager to kill in warfare as you might think. Especially when the other guy was compelled at gunpoint by his government to join the military (that is, drafted) just like you were.
For stylized cattle raids (e.g. historical Ireland, before the vikings) among old neighbors, there will of course be agreed levels below extermination (or the neighbours will be gone long before western contact).
Detachment is pretty clearly a (potentially) useful trait for a leader. Same thing for stuff like charisma.
For example, a leader might be good for their country at the expense of the international community. Or a senator might be good for their state but bad for their country. Or an activist might be good for his community but bad for society at large.
Such a person probably wouldn't be popularly seen as a shirking parasite. The subgroup that benefits would probably say they were doing a super job.
(Your argument also miss that psychopaths manipulate and damage the direct environment around them most.)
Indeed, if psychopathy is a combination of traits ('feels no empathy' and 'good at faking empathy') perhaps the survivors had a combination of traits that helped them to survive, while those with one or the other failed to survive.
There are large effects on behavior of psychopathy, people that are affected by a psychopath will mostly -- at a minimum -- make a point of warning everyone else they know. That is very detrimental for the psychopath. (Note that how we communicate really applies evolutionary pressure. Just check the number of muscles in a human face compared to most anything else. This is not a small effect.)
Also, psychopathy seems to be partly environmental and partly from genetics. And exist in most populations.
All this suggests an evolved behavioral strategy.
Researchers need to check for your hypothesis (the large different effects almost cancels each others out in many types of historic societies(!), so it is just a random genetic drift) since afaik it is a standard hypothesis, but it does seem very unlikely.
As for the advantage, it seems pretty clear to me: higher chances at being the alpha-male(female?), plus the resource advantage due to not having the ability for reciprocal altruism (and the ability to get away with that).
All few human psychopaths I have personally met were natural-born leaders. One is now occupying a high-level position within a successful company that produces manupulative F2P games. Another was a woman who, despite her being deaf-mute, was able to organize a successful remote web studio (before it fell apart due to her politicking and other members, including me, bailing out).
Also, quoting Wikipedia: "In Mongolia alone as many as 200,000 of the country's 2 million people could be [Genghis] Khan descendants".
Edit: if I remember correctly, Richard Dawkins' The Selfish Gene discussed cheaters (that is, individuals who didn't reciprocate) as being a viable evolutionary strategy, as long as they are in a minority.
Every now and then someone is born with one of their balances in an extreme position. The genes that express the balance either missing or just heavily surpressed.
It is not necessary for every kind of gene expression to be beneficial to mankind in general, as long as the average range of gene expressions are beneficial to our survival.
tl;dr: A certain amount of variation in genes is beneficial to survival of our species, extreme variation (like psychopathy) is less relevant (unless a psychopath wipes out all of us, which is a commonly known weakness of evolution in general)
The second thing to keep in mind is that many traits form a spectrum (particularly psychological traits). While the endpoints of the spectrum will likely be very problematic, there may one or more areas in the middle of the spectrum which are advantageous. Furthermore, it may be advantageous to have a variety of the trait in a population.
If psychopathy is genetically linked to say, a better resistance to cold or hunger, then it might stay even with the disadvantage of psychopathy, just because people with it will survive better.
Being a sincere participant in social groups has probably provided an advantage for just as long, if not longer.
Yes, and there still is. Evolutionary survival is not about living long; it's about passing on genes.
Psychopaths are great at reading people and exploiting emotional weaknesses. Humans are not naturally monogamous, and psychopaths are perfectly equipped for high-frequency sexuality. They've had Game for millions of years.