I am not sure what happened before this. But, police know that the woman is in a mental health crisis. The police break through the door. While she is holding a knife, she is not threatening anyone. Police officer clearly jumps and shoots. Why have the gun out first when you have tasers?
I don’t know the details of the case but:
Tasers are not always effective. They often don’t work on mentally ill and people on drugs.
During training all police officers have to watch a video titled “Surviving Edged Weapons”. The training shows how a person with an edged weapon can inflict fatal wounds to anyone within 21 feet before the victim is able to unholster a gun and defend themselves.