John Adler over at the Volokh Conspiracy has a post about an outing, where one blogger outed another blogger who was using a pseudonym. I'm not crazy about the practice of outing people who want to remain anonymous, and I like to think that it is for other than selfish reasons. Outing someone seems to me to be mean-spirited. The only reason to do it is because you want to cause some harm to another person. In that sense it is no different from humiliating someone or spraying them with water from a passing vehicle or punching someone in the nose who can't punch you back.
The level of harm in the cases is sometimes small, but it is still harm. Being good involves more than just not doing things that are really bad like rob, rape, and kill; it is also doing unto others as you would have them do unto you. And you wouldn't want someone to reveal your secret identity (if you had one for some reason), so you shouldn't reveal theirs.
Adler also says that he thinks pseudonyms can lead to bad behavior because you don't have to take the social consequences. I doubt that this is a big factor. The thing is that most people don't really think that their behavior is going to come back to haunt them anyway. The reason that we don't call someone stupid to their face is not because of fear of social consequences (I claim) but because of social inhibitions. When someone is standing right in front of you, looking at you, he is a human being, and you feel a natural inhibition to insult him just as you would feel a natural inhibition to hit him. Of course you can overcome such natural inhibitions, but most of us, in normal sitations, don't.
Then along comes the internet, and you are talking to a computer screen. And the computer screen isn't standing there looking at you. It doesn't have the audiovisual cues that create the inhibitions against conflict. Without the inhibitions, your behavior is controlled entirely by your judgment. And we all know that sometimes our judgment isn't what it should be.
What makes matters worse is that there is an asymmetry. You don't have the cues to prevent aggression, but you do have some of the cues which inspire aggression. You can feel threatened or insulted without having the person present who did the threatening or insulting. So you have the impulse to react aggressively without the inhibitions. Combine this with the fact that it is so easy to write something that appears insulting even when you didn't intend to be insulting and it is easy to see why so many internet conversations spiral so quickly out of control.
And that's why you should never hit the post button when you are upset.