The video evidence looks bad for the officer. I try not to convict until I hear everything, so I'll just say it appears to be a bad shoot.
Without relieving the officer of his responsibility, I do wonder how many commands to drop the knife and get on the ground were ignored by the deceased. It's all well and good that the officer be convicted if found guilty, but that doesn't really help this young man. When a cop tells you to drop your weapon, it's a pretty good time to comply...elsewise very bad and very permanent ramifications are sure to follow.