I think it would be a nice way to change things up for the police to save someone from death WITHOUT killing someone else.
How do you suppose this cop was to do that without killing the attacker? She was mid-swing with the knife headed toward the victim's neck. Nothing short of brilliant, accurate, decisive, gun shot would have saved that girl's life.
And the dead attacker had her whole life ahead of her, her school principal said; had it all written out how her life was going to go. I guess the little wannabe murderess just forgot to put in murder two people and go to prison for life as the first two bullet points. After all, she still could have her college education and even entrepreneurship while in prison. Cop just cut it short and deprived her of her planned life as a prisoner.