In my opinion, this is one of the downsides to the relative anonymity of internet as a substitute for other forms of social interaction. I think people routinely post things that they would never say in person. Sometimes common courtesy and a willingness to just live and let live are too much to ask of some folks.
Sometimes folks are just trolls - they get some glee from causing a flamewar. That's a shame, but there isn't much to be done for it. Full time moderation of all posts would be an onerous task. A lot of folks around here seem to be good at recognizing trolls and calling them out. If all of us can avoid taking the bait and responding to or trying to rebutt the trolls, they'd give up sooner.
Sometimes folks read too much into what is posted and see an attack when none was intended. Part of the limitation of this form of communication. This is less formal than people were when writing snail mail letters and it can lead to misunderstandings. When we type like we speak we can forget that nobody can see all the non-verbal cues that are associated with speech and add meaning. Sometimes all the smilies in the world don't make it clear.
All that said, these forums are far better than many I've seen.