You need either a strong sense of public duty, a wish to protect the weak and bring offenders to justice - or you need to be on a power trip, seeking the status and position from which to abuse victims.
Bang on @Babdoc.
Power is inherent in being a police officer (in a way that it isn't in nursing, for instance).
That is attractive to many- particularly people with a weak sense of self, or who rely on power over others for their own self-esteem.
It is a small journey from wanting to "protect the weak" to feeling disillusioned by the lack of status/respect for your "sacrifice" to then going on a power trip.
What is required is, a lot more weeding out at entry level, and a lot more monitoring of front line staff, by insightful, emotionally intelligent supervisors.
That's true whether you are talking about the Met, Army, prison officers, medicine, Church etc etc.