On a (perhaps unconscious) level, a person is very angry. If the person is a child in a hostile family environment, instinct alone would tell the child not to act out in anger against the family -- to do so might mean injury or abandonment. But we are like pressure cookers and can only hold so much in; the pressure must be released. And so the anger is released, but at a safe target -- one's self. This is the root of at least some self-injury, and a lot of negative self-talk and self-hatred -- in my opinion, anyway. (I hurt myself in the 1970's, by the way, when I was in high school. There was no help back then; I eventually just stopped doing it.)