Those letters are written by someone who hates herself and the world around her. Words of violence, hatred and anger meant to show just how tough she thought she was, when in reality she was probably a scared, self loathing teen trying to fit in with a peer group that most of us would shy away from. A peer group that is full of social misfits, deviants and, most often, sexually and physically abused individuals who direct their anger outwards at others around them.
I wonder if she's been diagnosed with Fetal Alcohol Syndrome? Might explain some of the reasons she has done the things she has- doesn't justify any of her action though.