If it were me, I would have walked a bit faster to get there and wouldn't have waited for the officer to scream "Shoot him!!" several times before I fired. I also live in a state with civil suit protections. No criminal charges, no civil suit possible.
Such was the topic of more than a few nightmares, involving my LEO father, when I was a kid so...
As for being shot by his backup... They are coming full lights and sirens. You should know they are close long before you start catching rounds. That being said, adrenaline is a bitch and can definitely f*** with situational awareness.
Either way, if I just stood there and watched someone get murdered I would never be able to look in my son's eyes again.