Someone shooting at the police is a clear and present danger to everyone's life and needs to be neutralized as quickly as possible. "Letting her shoot all the shells she had" sounds like a cute scene in a movie, not reality. You don't know what the shells will be able to penetrate, whether bystanders in nearby apartments or on the street are at danger, etc. The preservation of her life stopped being a concern when she pulled a deadly weapon and most certainly when she opened fire.