My guess is she was less than in the moment of expecting him to charge her. You can't see it in the vid, but the story is that he lunged at her with another knife he had in hand.
I'm not a cop to say what procedure is, but if I were her, my gun would have been in my hand the upon approaching him. I mean, when you see people running away like that, shit's happening, be prepared or be dead. I bet she doesn't make that mistake again.