People can be jerks, particularly online where they feel more anonymous (I guess). People with social and/or mental issues may stray far beyond being a jerk if the muse moves them. The recent ghastly shooting of a local news crew by a "disgruntled" former employee is a sickening example. The perpetrator had arranged to record his act and post it online; which he did, and even though the videos were quickly pulled, of course they were already "out" at that point.
By the way, we need to come up with a better word than "disgruntled" - to me that means you might let the air out of your former boss' tires, not start shooting people.
NYTimes BlogAs Farhad Manjoo writes, “The horror was the dawning realization, as the video spread across the networks, that the killer had anticipated the moves — that he had been counting on the mechanics of these services and on our inability to resist passing on what he had posted.”