Well, she's a abusive sociopath, so it's kind of natural that she doesn't stay in the highlight for too long.
Anyhow, as for the back story of her, it's almost certain that she has some troubling experiences in the past that are involved with interpersonal communication.
To me is seems almost like she is unable express any of her feelings too well, so the abusive behaviour is kind of reasonable behaviour pattern once you think about it.
Anyhow, I don't feel that enjoying the pain of the others is really that uncommon human trait. It's almost as satisfying as eating and the hunger definitely grows along with the hunger for power.
The deal with Mayo is that she only act in such direct manner because she isn't that skilled in other fields of life, I think.