Mistakes happen, and if that is all this is -- the tragic outcome of a best effort to protect innocent life -- then that is just the way it is. I wouldn't blame the LEO any more or less than I would blame anyone in that situation (a key thing in a world where we all know a non-LEO would receive far more scrutiny and blame).
But, some of you want to go further. You want this to be an outcome that is inherently justifiable merely because the LEO was defending his life. That is too much. Nobody should necessarily be free to kill innocents to save themselves. It can happen. It can be justifiable depending on the situation. But what it cannot be is always and everywhere acceptable, a component of policy or training, or some other absolute. Being on the receiving end of a life-threatening force is not a blank check to do anything at all when innocent life is involved -- not for police and not for anyone. So enough with the ex-post rationalization and defense of the blue line. This is what a number of posters here are responding to -- the extreme defense of killing an innocent apart from the details of the situation.
If one knowingly walks into a deadly situation then one must accept some risk. I don't want anyone coming to save me if their rules of engagement are "**** the innocents" the moment they feel threatened. As an innocent, that is more trouble than I need, and certainly far from helpful in a variety of situations. I would never ask anyone to save my life, to lay down their own, or to be a hero on my behalf. Nobody owes me that, and even if they did I wouldn't ask for it. What I am completely comfortable requesting is do-no-harm. This girl's odds of survival went from unknown to zero because help arrived. As noted above, that may be the tragic result of the best effort the officer could bring. But it is absolutely not fine for anyone to assert that the outcome is acceptable in general.