This is a pretty black and white scenario, so you'd probably be OK...
The standard for the use of lethal force is: Imminent, otherwise unavoidable threat of death or grave bodily harm.
Looking at the situation, you have clear ability of the rifleman to be a threat to the public.
You have a clear manifestation of intent, and opportunity is a given with a rifle & ambush position.
I'd say rifle-ambush guy has signed his death warrant.
That said...at least in my area, I perceive law enforcement as very "anti-assistance" beyond 911 and being a good witness.
Further, I saw the aftermath of a shooting incident in my city with a swat callout for a guy with a rifle.
The police shot their own car, and failed to mention that fact in the report. The forensic examination from another department brought that to light, and it was generally a comedy show.
A comedy show with rifles...and I have not seem or heard anything to make be believe the competency of the police has improved.
So...911 called. Witnessing like a 'boss. Vanishing like a 1980's movie ninja.