People are only people, and there will always be arguments between them. We all had rants and quarrels, sometimes unintentionally offending someone as a result of misunderstanding, bad joke or something even sillier. It happened to me a few times. In that case there are good manners and common sense to the rescue. But when it's intentional, there's no excuse.
http://www.bullyonline.org/related/cyber.htm