I've read that one major cause of people's feeling free to be rude, and even hostile and aggressive, online is that they can be anonymous.
I tend to think the person who is rude and hostile online is probably a person with a lousy disposition who hides his negative attitude toward other people in person (to some degree or another). People bring their "issues" to the Internet and either forget there's a human on the other end of the discussion or else enjoy being able to hostile . They're either "wilting flowers" in offline life and like the chance to share their secret, unpleasant, attitude toward people; or else they're bullies in offline life and just become verbal bullies online.
On top of that, a lot of people have trouble grasping the idea that a person who has an opinion they don't agree with isn't necessarily a monster. Online, the second, say, a Liberal expresses that he may have any number of people attacking him for his beliefs. The same goes for Conservatives, people of any particular religion or no religion, women, men, teens, old people, people who like Santa Claus - anyone. People are not interested in understanding other people's points of views or even in accepting that people who thinking differently may be perfectly kind, decent, caring, people. In other words, people with an attitude problem in offlline life can have an even worse attitude problem online.
I'm the same polite person online that I am offline; and even with that - oh brother - all it takes is one little opinion and you get people calling you names, calling you "crazy", or otherwise acting as if you're monster! LOL You just have to laugh it off.