Urban Dictionary defines a troll as "One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument".
The etymology of this internet slang term seems to be a combination of the noun and verb definitions for the actual word "troll". The verb to "troll" refers to a fishing method where you let the line out with bait on it, and pull it behind your boat; one prominent feature of the internet "troll" is that he baits groups of people into arguments, often disingenuously, by "trolling" a controversial statement behind his proverbial internet "boat" and seeing who bites. And of course the noun "troll" refers to an unintelligent, ugly mythical creature that dwells in caves and under bridges and who has a penchant for disturbing unsuspecting passersby, so the correlation there needs no explanation.
The slang term has utility when applied to those who do, in fact, disingenuously attempt to stir up controversy and raise people's ire just to amuse themselves, or for some other end. For some reason, Rob Bell just leapt into my mind. But, like any pejorative term, "troll" can often be used excessively or inappropriately as a way to demean and dismiss those who disagree with you without having to actually engage them.
Moreover, genuine statements of honestly held opinions can be made with some desire to be provocative -- but not for the sole purpose of being provocative -- and I see no reason to classify these actions as "trolling", even though they closely resemble each other.
If there is some prevalent sentiment that someone genuinely disagrees with and they want to make that known, what else do you expect them to do? Simply not speak their mind? Happily accept the label of troll? What have they done to deserve such a label? The conundrum is obvious; any unpopular sentiment voiced to an unsympathetic audience, even if not for the express purpose of attempting to get a rise out of people, is often indistinguishable from trolling, from the perspective of the unsympathetic audience. At least at first blush. Within a community of posters or commenters, given enough time, it becomes easier to recognize those who like to raise legitimate points of concern or disagreement from those who simply enjoy annoying people by being confrontational and disagreeable. Outside of consistent online communities of known members, though, what sometimes appears to be trolling can occasionally be simple, honest dissent from a widely held view.
So I suppose this isn't a defense of actual "trolls" -- it is just and righteous to scorn and shun their actions at every opportunity -- but of those who sometimes are incorrectly labeled trolls because their actions can appear identical to those of true trolls.