River is a complicated character and Steven Moffat does not shy away from her roots as a psychopath trained to kill. She manipulates the people around her with little regard for anything beyond her goal at the time. Similar to the Doctor she is a trickster, the one who arrives to shakes things up leaving chaos in her wake. She exists for the sake of the adventure and any trapping beyond that is beside the point. What keeps River from falling into caricature is her underlying vulnerability. She is a tragic figure but while she is sad at times she does not feel sorry for herself. She cares deeply but not broadly and the implication is that when River and the Doctor are together they are both made better for it.