It still would of ended tragically. The knife she was holding was in the midst of going straight towards her neck. Assuming the cop ran forward or even tazed her to stop her, you still have to consider which would have landed first. The kinife, the police, or the prongs. I think the knife would of landed first. With adrenaline pumping and blood coming out the target would be dead by the time the ambulance came. Instead of being dead she would be in prison for murder. Even if no one died, she would be charged with attempted murder or aggravated assault.
The whole situation is tragic, before, after, and during. It is wrong to place the blame solely on that police for her death. Many people failed her.