I was born in Rio de Janeiro state in Brazil. The people from my state are usually fun and friendly people to hang around. They enjoy a conversation and having a good time, really approachable. Until you talk to any of them on the phone.
After I moved to São Paulo, one of my first jobs was on a call center doing tech support for one of the countries largest ISPs at the time. This is when I learned to loath the people from my own state. Once they're on a situation without face to face contact or the possibility of being smacked, they turn into the biggest assholes around. Impolite, arrogant and stupid assholes. Specially when they believe it's their money that pays your wages (hint, it was not. 2 million costumers paying 40 buck a month, each one contributed fraction of a penny to my wages).
Fact is, unless people are educated (I would even say "indoctrinated") from an early age to be polite and respectful even when not in presence of the interlocutor, people turn into beasts, simply because they can (i.e. there will be no consequences).
Someone should do a research (if they haven't already) on humans ability to empathize while separated from others. The results would explain pretty well why internet trolls act like they do.