I'm not sure "mistrust" is an accurate word for a desire to see someone dead.
Jesus fuck, do we need a cartoon drawing?
If she 'mistrusts' the cops, say she has a personal history of seeing cops abuse their powers, infringe the rights of suspects, and have quick trigger fingers in certain neighborhoods, then she may distrust their motives in shooting him, or their reports about what happened.
If she perceived that the cops had killed him when they didn't have to, she may have felt a need for revenge to be visited upon those that did it.
In the heat of the moment.
On a false premise, or an emotional outburst.