She only killed Nusrat because she caught Bagga trying to strangle Mira, so she had to get rid of the witness.
In the film’s biggest WTF moment, Mira discovers that inspector Bagga is really the one who killed the private investigator Walter (Richie Lawrie), tried to kill her several times, and killed Nusrat.
Bagga has been trying to ruin or kill Mira for a while.
Bagga is the daughter of a man that Mira (as a lawyer) got put away for various crimes.