I think this song is about two best friends who have been friends for years, but one day one of the friends girlfriend is cheating on him with the other friend. and then the actual boyfriend of the girl (friend 1) secretly finds out about the affair with his best friend (friend 2) one day.

"Wait until it fades to black
Ride into the sunset
Would I lie to you?
Well, I've got something to say " this is when the friend 1 assures friend 2 that he wouldnt lie to him in a test to see if friend 2 will tell him about the affair.
"Grab your six-gun from your back
Throttle the ignition
Would I die for you?
Well here's your answer in spades " that's when he goes to extra precautoins and tells friend 2 that he'd even die for him, in a hope that he will fess up now.
"Shotgun sinners
Wild eyed jokers
Got you in my sights
Gun it while I'm holding on" this is when friend 1 starts going crazy and thinking about killing his "best friend"
"After all is said and done
Climb out from the pine box
Well I'm asking you
'Cause she's got nothing to say" this is when he actually confront his bets friend and asks him, showing that earlier he asked his girlfriend and she lied saying it wasnt true.
"The angels just cut out her tongue
Call her black Mariah
Would I lie to you?
That girl's not right in the brain " this is when he lies to his friend and says that his girlfriend confessed about the affair,"That girl's not right in the brain" is showing friend 2's excuse saying that's she's crazy and she's making it up, which makes friend 1 even angrier since he's not telling the truth.
"Mass convulsions
Strike the choir
By the grace of God
Gun it while I'm holding on" this is when he finally loses it and one day gets a gun and sneaks up to his friends place where he knows that his friend and his girlfriend are having sex, and is going to shoot them. "the choir" is his friend and his girlfriend "choir" = the sound of them having sex. He thinks its gods wish that he shoots them since they have sinned.
"But don't stop if I fall
And don't look back
Oh baby don't stop
Bury me and fade to black" this is when he tries to reassure himself that he can actually do it, and not pull out. Trying to convince himself that its for the greater good.
"She won't stop me put it down
She won't stop me put it down
She won't stop me put it down
So get your gun and meet me by the door" this is when he's actually pointing the gun at them both and he starts to freak out, he doesn't want to shoot his girlfriend since he still loves her, but he doesn't want to back out either, so he tells his best friend to duel him for her, even though his friend doesn't want to, friend 2 realises that there's no convincing him that there's other ways of settling it.
"Don't stop if I fall
And don't look back
Oh baby don't stop
Bury me and fade to black" this shows when theyre both outside facing eachother, guns in their hands. The final show off to settle all matters.
how it ends is up to your imagination