There is more than one reason why people are rude, abusive, insensitive, defamatory etc, etc, on the internet. But I'd hazard to guess that it comes down to two basic reasons.
1. They are anonymous so they don't have to worry about any
awkward reprecussions of a physical nature. They're allowed
to express how they actually feel about a topic or person and
still keep all their teeth. So it works well for them.
2. And why do they feel the urge to vent such hostility? I think it's because in the average persons daily life they have to take a lot of crap from a lot of people. Teachers, bosses, cops, spouses, parents, siblings, etc. At the end of the day quite a lot of resentment could be built up. Chat rooms and message boards seem to be a perfect outlet for releasing all that stress and pent up anger.