The sort of behavior you allude to is not restricted to any one ideology, it is more of a psychological trait, or perhaps more of a social pathology.
A concerned and idealistic individual has an insight, or perceives an injustice and is desperate to correct it, but has neither the resources nor the opportunity to do so and instead tries to compensate by proselytizing his outrage to anyone who will listen. Those who disagree are then identified as the enemy, or as mindless dupes and spineless cowards if they simply fail to share the enthusiasm. Life then becomes an endless struggle to convince and persuade, and failing that, to humiliate and persecute the opponent. And when you lose your temper and tell them off, they accuse you of refusing to discuss freely and attempting to stifle their ideas.
Suddenly, enemies and conspiracies are everywhere, and those who are believed to be part of them are only slightly more evil than those who simply don't care, or if they do care, choose to limit their involvement. When several of these true believers get together they reinforce each other's self-righteousness and a feedback of outrage spirals out of control. The politics or the issues may differ, but the symptoms are remarkably consistent whether the angry young men are either left or right (or religious fanatics, or anomalists, or animal rights people or Hare Krishnas, or racists or whatever). There is no shortage of causes.
I'm an atheist, but I recognize there is nothing more annoying than a doctrinaire Madalyn Murray O'hair-type professional atheist. They simply will not give it a rest. During my college days, when conservatives where scarce, it was the leftists who were insufferable with their endless committees and pamphlets and demonstrations and desperate attempted coffee shop conversions. And of course, they were always spouting their twisted scripture. And if you failed to agree with their program on any point, however minor, they simply would not leave you alone, and if you refused to accept their extremism then they accused you of fascism. One friend of mine, a devout Marxist himself, expressed it perfectly: when I asked him why he didn't join the Party or some similar organized group, replied, "Just because I agree with their politics doesn't mean they're not assholes."
It's sad, because these individuals are usually operating from the best of motives, and are often capable of genuine personal sacrifices and effort to promote their cause. But they wind up alienating their friends and driving off their more moderate or less zealous aquaintances, or otherwise enraging everyone around them. And they always seem to bring out the worst in their ideological opposites, so the cycle festers and feeds on itself, and the voices of moderation and reason and tolerance are progressively silenced, and those who would prefer to speak softly are driven away.