Looking at the tape, this is one where the officer has to make a split second decision. He doesn't shoot odds are the girl with the knife kills the other kid. It's not at all like kneeling on a guy's neck for 9 some minutes.
Again a split second decision. He either does something or the girl stabs another kid. This is why policing is a tremendously difficult profession and why I stand behind law enforcement. The taser may not have stopped it, he could have been stabbed, etc.