There is something that makes intelligent (questionable) adults behave like assholes as soon as they sit at their PC. I think some people have this emotional baggage and the internet (the machine in the corner) is sometimes seen as a safety valve to let off steam, to release this internal hatred. Here on the internet, people who met only five minutes ago can be trying to exchange bodily fluids (cyber sex or arrange meetings), while people who met six hours earlier are screaming a whole lifetime of hoarded hatred at each other like they want to kill each other.
It costs so little to have an opinion about anything on-line and because the words they type comes on to YOUR screen it makes it oh so personal and it's not only the 6ft tall muscular gangster types, it's also the skinny 14 year old acne faced boy who is delivering more curses at you as fast as the data packets can reach you and that's where the answer lies I think. Because you can do it anonymously and irresponsibly you can become almost "drunk" on the power that you can say anything you want and get away with it. There are other influences which might better explain it, the most obvious being TV.
These people maybe think they are being witty (cue taped laugh line) and that feeling is reinforced in chat rooms where some of their supporters will laugh sycophantically with them and at you. The internet gives them that studio audience they crave. How easy is it to react aggressively when your personal space on your PC screen is violated by someone else's opinions and almost licensed scorn?
People are assholes on-line because they are assholes off-line or because they want to be assholes off-line, but don't have the nerve, and quite possibly also because no-one will listen to them. I don't consider myself a bad person, but I certainly write with venom sometimes at these morons when I encounter them.
So sue me for being human!