She spirals

A police officer is given a partner. A girl who was held in a maximum security asylum. They hunted down the serial killers. Who better to understand a serial killer then another serial killer?

When the officer finds herself falling for the insane girl she tries to ignore it. Decides to switch her department.

Unknowingly she sends the girl spiraling. She becomes worse. She goes on hunting sprees. Appears back at the station with blood covering her constantly.

Now I'm getting ahead of myself. This is based on how the officer comes to terms with her feelings. And how she helps an insane girl function in society.