High conscientiousness correlates with agreeableness; it's rare to have a highly conscientious, highly disagreeable person who would actually go out other way to offend others. So I don't think that's that's the explanation.

Further, commentors on any particular subject are, I think, statistically more likely to be monomaniac obsessives, and that correlates with poor social skills (e.g. truth-seeking at the cost of empathy). In other words, autism.