I would ask 1. were the officers in range to use less than lethal force, i.e. pepper spray or taser. 2. was there any verification that they told the youth to drop the "rifle" 3. was the youth actually holding the rifle in a "firing" position or did it just happen to point at them due to the way it was slung/carried.
On the surface, I may tend to support he officers depending on the answers to the above questions. Now obviously if this were a civilian, not a LEO involved in this shooting, there would be an arrest, but since it was LEO's, the there won't be any. I know many (including me at times) think the police use excessive/ unwarranted force but at the same time as a prior security officer I have been in situations where I had to determine a possible threat on very little visual information as well. Thankfully for me, I never had to fire in self defense, but I have had to present on at least 2 occasions.
This case does have a bit of a fishy odor about it, but based on the facts presented, I would tend to side with the police on this one. Let the BBQ of me begin because I don't automatically find the blame on the police.