Yes, all of the above. Narcissists are a dime a dozen and don't pose any threat. I've encountered more of them than I care to count but I view them as pathetic and sad. It's the sociopaths and psychopaths you have to take seriously. Between the basic lack of empathy and compassion of the former and the innate ability to inflict harm upon others associated with the latter, you're basically talking about most of the serious criminals and predators that prey upon society. Not every sociopath acts upon his or her warped views of humanity but they're certainly capable of doing so. A true psychopath is obviously a potential threat and should be viewed as such.
As to stories, a prime example of sociopathic (even psychotic) behaviour was exhibited by someone I had the misfortune to encounter right here on Yahoo! Answers. This was years ago, before the site upgraded and addressed serious security loopholes. I'm not sure how long you've been an active member here but there was a time when a person could literally stalk you in this little asylum. There was no measure for blocking or preventing other members from accessing your Q&A. So what the creep did was create upwards of 30 different accounts and reign "terror" on my entire Q&A by simply attacking each and every post, whether question or answer. It might not sound like much but he was able to successfully rig the voting every time a question I answered went to a vote.
For example, I'd answer some obscure question that no one else answered (or maybe one other person) and there would be 27 votes for the other answer (or *no best answer*), no matter how weak it was, and 1 or 2 votes for mine. As you might surmise, he also went mad with the *thumbs down* feature. And it wasn't just me. The psycho was also able to access my contacts list and went after some of my best friends on here. It really made this place a misery, and the Yahoo! Answers Team did absolutely nothing about it at the time.
I ultimately left the site in disgust and most of my friends joined me in protest. It just wasn't fun anymore and Yahoo's apathy didn't help the situation. I only returned recently. Most of the people I used to correspond with on here are long gone. I can hardly blame them. I'll be interested in seeing how much things here have really changed. So far, so good.
In conclusion, I think the psychos and socios out there are attracted to the internet, particularly community sites like this one, because the veil of anonymity allows them to act out their sick fantasies on an unsuspecting person or persons. I don't know, perhaps it's better that they get their kicks victimizing people in the cyber world instead of the real world. Good question, btw. I wonder if any socios or psychos will answer and give us some insights as to why they are the way they are.