First of all there was no imminent threat to the cop. He should be charged with first degree murder. These guys are trained to recognize that. If they fail training then they shouldn't be a cop, period.
Secondly, it took a year to come out because of political reasons, pure and simple. Rahm Emanuel is as corrupt as the day is long. He is a democrat, need I say more?
1st degree murder is usually premeditated, which I think you’d have a hard time proving in this case.
As for training, maybe you should read the article (the author happens to have decades of it).