That's something I think people forget; the vast majority of folks don't have some kind of split personality disorder, and their behavior online is the same behavior they exhibit in real life, except perhaps toned up a bit.
We all sometimes have the urge to do or say inappropriate things, no? A big part of what makes you, you know, you is the degree to which you have a "filter" that screens out those inappropriate urges before they turn into inappropriate actions.
This filter is rather different in different situations, because what is appropriate (and what has consequences) is different in different situations. In a very real way, I would argue that changing that filter enough (and pesudoanonymity is usually 'enough') makes you, essentially, a different person.
Most people, in a pseudonymous situation, have much less of a filter, in part because bad behavior is more acceptable, and in part because bad behavior has fewer negative consequences, both to themselves and others, than it does in a real life situation. (I'm not saying that there are no consequences; people are hurt. But usually not as much as people would be hurt if the same people removed the same filters in real-life interactions.)