I've known a few online, on several fora. Without a doubt they had some seriously sociopathic characteristics. I have quite some stories about them.
I saw much of the same traits as PricklyPorcupine noted above. In addition, they are ruthless in going after anyone they suspect has their number and has sniffed them out, including recruiting toadies to do their dirty work. They surround themselves with subservient types as personal foot soldiers in their power play. They use their interactions to gather ammunition to know exactly what buttons to press. Masters at character assassination games and knowing exactly how to trip up, bait and trigger their targets. They love backstabbing, publicly humiliating their targets, and playing one side against another after turning on the charm.
Here's an example of a tactic they might employ: Suppose you have been talking to a woman who they know has had a stalker in her past, and thus is wary of stalkers. In the guise of a "heads-up", they would do their best to paint you as a stalker in her eyes, intentionally triggering her and pandering to her prejudices while feigning friendship with her, having gathered the requisite ammunition from her to use against you using her as proxy. Then she'd be goaded into backstabbing you as their toadie, while her penchant for gossip would also come in handy as a foot soldier in character assassination. Such common tricks are pretty much military-grade tactics, straight out of Machiavelli and Sun Tzu. The clever thing is that even if such games are known, that doesn't dissolve the suspicion.
A common disarming tactic I noticed is to ridicule another's behaviour (such as laughing at how another person tends to lash out at people who fail to boot-lick them), before strategically employing the exact same behaviour themselves towards you - then accusing you of doing what they are doing! (They might then defend the very person they ridiculed to you and turn them against you, after charming them.) The latter generally has the desired effect of putting one on the defensive, which they can easily spin as evidence of emotional instability as a discrediting tactic to further their character assassination games. Then those on their sides will deny they are capable of that and accuse you of slandering them for pointing out the facts. Then their allies will take their slander of you as fact. In effect they get others to project their own unsavoury characteristics onto those they target. They will usually cap it off by getting everyone to think you are playing the victim and thus attempting to manipulate them!
I've seen all the above and more in action on the Internet. It's not pretty. Then again, the sheer cunning of it all is mind-blowing. They basically know how to play everyone's weaknesses like a fiddle and amass an army of sycophantic followers, who are hugely impressed by their superficial charm, status, confidence, power and supposed strength. Which of course is immensely gratifying for them.