How mean is the real mean girls?

I mean, the one you encounter in schools or in work? Are they really evil, or there is a little good side of them?
I also have encountered some mean in my life, they are really mean. At first you would think they are your friends but in the end there true color would surface. You'll be surprised to know that they are very good backstabbers, literally they are good in character assasination.
As a human being I also think they have a good side of them, but it is as little as a grain of rice.
I really wonder what drives them to do all these things. I think that these girls have low self-estreem, then in order to gain psychological security, they would put a person down.
What do you think? Are there any causes for them to be mean?