First off, I am assuming there are witnesses to this. Otherwise it could definitely be murder. Could you imagine explaining away how you accidentily shot your wife at home with what turned out to be a lethal round?
I wonder if the officer just pulled his weapon out and fired or if they spent a couple minutes talking about it, setting up their position where the recipient of the bullet was ready, waiting and braced for impact? If the latter is what happened, that "victim" should have taken responsibility enough for his own safety to be assured that he wasn't about to fired on by a live and deadly round.
The amount of negligence on the shooter's part should be enough to relieve him of his job I would hope even if it doesn't get him charged with a crime.
As far as comparing it to the teenagers. I wonder if the victim in that case was a willing participant in being shot alike to the incident involving the police officers.