You have no idea. I was "friends" with someone for years, who at the time I didn't know was a psychopath (over time I realized they met literally every item on the psychopathy checklist). The manipulativeness, the ability to lie completely sincerely (I bet a lie detector test would show absolutely nothing), and absolute and utter lack of remorse is terrifying, in a way.

The most striking thing, in retrospect, is the manipulative talent they have, and how relentlessly and patiently they are to go for the "long con".

In my case, I happen to be very talented at programming and computer science. The psychopath (who has no particular scientific talent) I refer to found me, and basically befriended me and over time convinced me to work some 100+ hour weeks on an entrepreneurial project building prototypes. This would ordinarily be fine if the profit was split evenly, but only in retrospect did I realize the disparity of my persuaded reality from their distortion field versus actual reality -- i.e. this person took 100% credit for my work, and was sure to leave no evidence that I did anything so there was nothing I could do.

But that's not even the persuasive part. The persuasive part was when I brought this up with my "friend", saying "hey so I noticed you're taking complete credit for this with no mention of me" or something of that sort. I don't even remember the details, but this person masterfully diverted attention in such a way that I found it not important, and thought about other things.

Ultimately over time, I noticed all the little disparities (lies, deception, hints of absolute lack of remorse) and had a pretty good suspicion that this person was a psychopath. It was only when I caught him in a completely undeniable lie that it all exploded.

The excuses given when confronted were pretty impressive at the time, but I'm not stupid, and it was pretty clear he was struggling to come up with answers when confronted. The strangest thing though was despite me having hard and undeniable evidence, to this day he would refuse to admit the lie, and in fact tried to convince me to apologize for slandering him (despite him knowing very well I have conclusive evidence).

Then I let on I knew he was a psychopath. The change in personality was dramatic, creepy, and like something out of a horror movie. You don't know this person, but normally he has a very persuasive, seductive (in a non sexual way), and extremely socially charming and polite personality. When you're around him, he makes everyone feel good about themselves (part of his strategy to get people to follow him and do what he wants is making them addicted to feeling good about themselves around him).

But after he knew I knew, things were different. The attitude DROPPED completely. He had a stone cold expression, no emotion in his voice, and the coldest eyes and voice tone you could imagine. There was only one emotion he showed when I revealed I knew, and it was terror. Deep, deep terror he was struggling to conceal. This made the situation all the more creepy, because this person is characterized by being completely fearless (as is typical of psychopaths), but I suspect if it's one thing that scares a psychopath, it's being found out -- which btw is why this article from the OP is so remarkable (though no doubt it's just an attempt at manipulation, if you know anything about psychopaths). But beyond the terror you could see, there was nothing -- just pure cold and calculating rational self-serving thought. He immediately terminated a multiple year "friendship" without even a thought, and distanced himself from me after he realized I could not be persuaded and distorted into thinking I was wrong and imagining things.

Since then we've crossed paths only a few times, and you can tell he's still afraid of being revealed. He's doing well though in business as you would expect of a psychopath, climbing the social ladder very quickly to a future of possible fortune.

I should add that it is indeed a common misconception that psychopaths are cold blooded killers. The reality is in some ways much more scary. The stupid ones perhaps kill, but they eventually get caught. The smart ones are leeches on society, who have no regard for anything but their own animalistic desires of power, control, and inflated ego.

Also, having been "friends" with a psychopath gives an interesting perspective on other well-known figures. Though I never met him, I am convinced Steve Jobs is pretty high on the psychopathy spectrum. What's interesting is that Jobs accomplished something valuable to society, though not without leaving a trail of damage in his wake. The psychopath I met also has a strong motivation to make an impact in the world, to be known almost as a hero, no doubt to satisfy desires of ego and power. So perhaps society's social structure can work for good and turn the desires of a psychopath towards good (but unfortunately it can also work in reverse). So in a way, I think psychopaths aren't all inherently leeches on society if what they accomplish is positively impactful to it.

But you see... the scariest part of having encountered a psychopath is it's sometimes very difficult to know if that last thought is my own, or something I was persuaded to think.