If Thoreau was right when he said that the mass of men live lives of quiet desperation—and I believe he was dead on-target—then right there you have all the motivation you need to account for why people project the kind of destructiveness Mike's OP is calling attention to. Especially, as noted by Kacey and Sukerkin, when shielded by anonymity. It's like road rage: people, safe in their car-fortresses, commit acts of unspeakable rudeness that they wouldn't dare do in face-to-face situations—and probably wouldn't even dream of doing in those situations. The key factor, I think, is that people are unhappy, probably because they feel that in critical respects they are powerless over crucial factors in their lives. And that gives them the incentive to try to exercise any excuse for power that they can come up with. Destructive power is still power, after all—it's cheap and commonplace, compared with the much rarer power that constitutes the gift of creativity; it's far easier to damage a work of art than to create one, eh? But a lot of people, in my experience, will take what they can get in this domain, and if all they're capable of is destructive negativity, they'll still do it. They're having their little moment in the sun, they're getting to have an impact... and for them, that's really all that counts. It's like the insignificant worm in Nashville who at the end of the movie shoots a world-famous country music performer, for no other reason than the desire to be famous. We've seen that in real life all too often, eh?
And it's not just MAs where you get this on internet discussion boards. I stumbled across a theology forum where the exchanges were as hostile and negative as anything we've seen posted by our worst MT trolls—but clothed in the language and style of discourse demanded by the culture of that forum. Still, you didn't have to do too much reading between the lines to see the contempt, hostility and sheer desire to make someone else look bad that was present in the exchange. All the witty sarcasm and would-be droll humor couldn't disguise the fundamental destructiveness of the posting in some of the threads.
It's never going to change, as long as there are unhappy people in the world who hate their lives, and transfer that hatred outward (since they can't really do anything with it internally).
IMO, there's nothing wrong with a really sharp, hard-edged debate and uncompromising criticism of weak ideas. We've all seen people who doggedly push ideas and arguments on behalf of those ideas which incorporate fallacious reasoning, factually mistaken claims, or unsupported background assumptions; bringing the weakness of such thinking to light isn't destructive—it keeps the bar raised to the necessary height that only the true story will be able to get over. Filtering out mistaken reasoning is a big part of why we know more know about how the universe works than we did a thousand years ago; it's crucial to progress. But you can tell in a heartbeat if the post you're looking at is aiming at that kind of critical vetting of other people's suggestions, or is instead simply heaping abuse on anything anyone else says—nastily and indiscriminately. The contrast is total. And when you encounter the latter, the best response is probably to heave a sigh of relief and pat yourself on the back for your luck in having been born who you are, rather than the sour, unhappy hostile person responsible for the post in question. Having to be that person is its own punishment.