I'll reply to both Larry and DJ: I don't absolve the cop, nor do I want to skip questions about what police are trained to do when a knife is drawn. I'm saying that this cop is probably trained to see the swinging knife (prop knife, butter knife, or other) as a lethal threat and made a decision in haste. By it's very nature, such a split second act will miss all the nuances, and insights into teenagers, and so on. Though it's impossible to say, his decision could have been influenced by beliefs about skin color and violence -- that's beyond the powers of human investigation, as we lack the ability to precisely recreate this exact scenario with this shooter and a white teen girl in some alternate universe.
If anything goes on trial, it would have to be what police officers are trained, and equipped, to do in such situations. And what discretion they feel they have, when children are involved.