Beria, most of us have to be restrained and careful what we say most of the time so that we don't upset the boss, work colleagues, partner etc. So my theory is that, in anonymous situations like the chatrooms you are talking about, people can let off steam without any repercussions.
I think it's the same on the road - people let their anger and aggression take over because it's relatively anonymous. If they behaved the same when not in their cars they would risk getting punched!