There are several incidents where LEOs (and Citizens) have shot perps in the back WITHIN that 1-3 secind window They usually are not charged.
I am no expert, all I have is an opinion. I find it hard to imagine a citizen shooting someone in the back and not being charged; regardless of the time frame in seconds.
I personally hope this officer is not emotionally and professionally scarred from this situation. No one goes into an uncertain area, meets a man with a gun and intentionally shoots him. It's unfortunate. The victim could have prevented his death (from what I could see).
This officer and his family needs prayer.