Hi far east.
Great question!!
In my opinion, most people have a big emotional investment in the views they express, and when someone differs with them, it is human nature that they take it personal, and that upset takes over the points being exchanged and discussed.
When you add to the formula a medium where the contact is not face to face, the harshness can flow more freely.
It is a challenge, and it takes effort to not take an emotional turn when people disagree. People slip and say things and take on a tone that is no longer about the exchange of ideas.
Personally, I dont look to disagree just to disagree, but when I find someone who disagrees with me, for example, I very much enjoy the challenge of learning their view and responding with more clarity in trying to be true to my view.
Again, it takes effort to not go the emotional nasty route based on someone holding a different view. Some people try and mature in their ability to argue a point, others dont try and they get frustrated. I think all of us have been on both sides of that fence.