The two parents were used. They were given protection by a race of rat/mouse people when they ran away from a human settlement (the male was going to be terminated for being a risk), but the only reason for that was so that the two would have a child. After the child was born both parents were killed and the child was raised by the rat/mouse people to be used to help bring down the human settlements. Humans have strong psychic powers in their time, but to prevent abuse a lot was done to weed out and prevent people who would turn their powers on other humans, including genetic engineering that would result in a person dying if they killed a fellow human. The rats raised the child thinking she was one of them and not human, so killing other humans wasn't an issue for her, while on the other hand the other humans could not willingly turn their powers on her to kill her and stop her from slaughtering them.