I almost lost it on reading this, since honestly if this was true we'd probably have seen society being ripped apart at it's seams before there was an Internet to get these people to vent through.
Let me be honest about Trolling, for the most part it tends to be entertaining, not just for the troll, but for people in general. In your average MMO for example you wind up with a lot of people doing repetitive tasks and getting bored. A good troll generally makes people laugh and is part of the community, helping pass the time and make a lot of the more annoying aspects of some of these tasks seem not quite as bad. Your typical troll, is basically just clowning around, and continues because he knows huge numbers of people are laughing with him.
The thing about racism, sexism, and all of these other things online is that many cases it's done largely to knock people off their high horse. For the most part it's because you wind up with a bunch of people, usually left wing, who decide to come in and start yelling at people for telling off color jokes, and making preachy social justice pronouncements that they simply assume everyone agrees with. A lot of times your "troll" is someone who simply comes out to knock these people down a few pegs, and ultimately get them to shut the hell up (or at least create an entertaining argument for spectators). This is a big part of why so few trolls are "brought to justice" as people put it, because anyone who reads chat logs will oftentimes find if they follow the back far enough that the troll has people laughing with them, or if they seem to be targeting someone that the target is typically someone who came rolling into a chat room and pretty much dumped an ice bucket on the entire thing. Indeed if you hang out in certain MMOs you'll find that specific trolls have fan bases and even occasionally wonder when a favorite will be back on to be entertaining (usually when they are about to sit down for some heavy crafting, resource grinding, or going to be spending a lot of time waiting in queues). In short, this might not be the most popular point out there, but so called "trolls" actually contribute to their communities more than most people do.
On another front, let's take perhaps one of the biggest trolling groups ever... "Something Awful" and their infamous "Goons" (I'm not currently a member of the SA forums, one of these days I'll have to find the time and put away a spare $10, I barely have time for The Escapist nowadays). If you've been paying attention you'd notice this is a very famous internet site, that sees incredible amounts of front page traffic. A lot of their trolling antics, especially in their "golden age" on Second Life and such, was done specifically so they could create articles and videos about it and entertain people who visited the front page... and taken in context a lot of it was pretty hilarious, especially when it was taken in context. A lot of the targets also tended to be those who kind of deserved it, or games with an intentionally disturbing presence, that ripped people off, or were known for some truly unjust egomaniacal moderation.
I guess there is some truth to Gabe's "Greater Internet Fuckwad Theory" that a normal person + anonymity + an audience = complete retard. However I don't think that it's the ENTIRE truth, I think for the most part it requires the right combination of positive audience reaction, and a correct game environment for it to work, to get trolls really "into" it. Sadism doesn't enter into it I don't think, especially seeing as nothing forces you to endure a troll, as you can always log out, change characters, or whatever else. In many cases if it seems personal, simply shutting up will usually do the trick because that's the point... that your ruining everyone else's fun by being a preachy twit and taking things too seriously, or whatever.
I will also say that trolling and cyber-bullying are NOT the same thing. For the most part Cyber Bullying involves latching onto a very specific target, and usually involves activities outside of what can be seen in the public. Usually by someone who knows the victim IRL, or has become obsessed to the point of tracking them (often illegally). It's a much bigger deal, but also much rarer. Say for example if someone decides to get online and pretend to be nice to someone and become their girlfriend/boyfriend online simply so the messages back and forth and confidences can be shared among people IRL to mock the person, that's akin to Cyber Bullying. If someone is just making off color jokes and being a bit digusting, or is mocking you because you logged into zone chat and started trying to administer your own brand of "social justice", that's entirely different, and honestly if you log off or just shut up for an hour or so that's likely to be the end of it. The troll doesn't want to hurt you, he's not going to say try and embarrass you with classmates or people you know in your RL workplace.
That's my thoughts at any rate. Saying that trolls are probably sadistic psychopaths IRL is a real stretch (which is what this seems to be implying).
At the end of the day, I think the big reason why trolling persists is because it's not that big a deal, and really I think those that get all uptight about it are the ones who are a real problem for the most part, their strong anti-troll positions largely being because they can't manage to co-exist online without making sure everyone conform to their idea of decorum and morality, and thus wind up becoming troll victims, or at least Sh@t magnets where people start acting unusualy racist/rude/disgusting around them just because they tend to be so preachy.
I can appreciate some other responders thoughts on this about how this could apply to the whole "video game violence" thing, but honestly, I think what's being suggested here represents a greater evil than even that argument. If you think about this, you'd pretty much have to say that almost everyone involved in SA's "Goon" movement is a genuine sadistic sociopath IRL, as opposed to simply accepting that the site is pretty much what it says it is... an internet comedy site.... and I'm just not buying that. I mean I doubt you'll walk into Lowtax's house and see a bunch of severed heads in the freezer next to the ice cream, and if you take this seriously as the sites creator/sponsor/ultimate authority (to the best of my knowledge) he'd have to be at least that far gone for this to make sense... he'd be kind of like Kevin Bacon and the Goons would have to be his "Following" IRL.