I personally wouldnt engage anyone in a situation where both parties are armed and shooting at each other. No way to know who is who or what is happening.
Every other situation would have to stand on its own merits as I witnessed then and likely I would need to have seen the start of the encounter. Im no hero, no cape etc but I would have a very hard time living with myself if I stood by and watched an innocent be killed or maimed when I could have stopped it. Keeping mind that stopping it may not involve the actual use of lethal force but prepared to do so if forced too.
As far as rural folks being surprised by LEO response times, if they are they must have just moved from the city. I was born raised and live in the hills of Ky 13 miles from the nearest tiny little town. A call to 911 for LEO even to a shooting in progress is about as helpful as a prayer for Wild Bill Hickok to appear.
They will show up. Eventually. In likely about half a hour or so.