The kid should not be running around with a knife. I won't lead to anything good and he is certainly not free of guilt.
But anyone who watched that video can see that that guy was not an immediate threat to their lives. He was not appoaching them. I was around a guy with a knive like that one time too, and though I was on high alert until the cops came arrested him (without lethal force) at no time did I feel the need to shoot the guy.
What I hate most about this, though, is that everyone feels the need to take sides in these situations. Both the kid and the cop were in the wrong. Why is it so hard to say two people can BOTH be wrong? It's not an either/or here. The kid was stupid, and so was the cop. End of story.