It's not that I strongly advocate judge, jury, and exectuioner. Don't get me wrong. It's that the BG posed an imminent threat to the victim.
The BG's intent cannot be determined with sufficient confidence to conclude that he was indeed making every effort to retreat. Let's start with some of the facts:
1.) The BG threatened the victim with death.
2.) The BG was armed, and further armed himself with the victim's shotgun.
3.) The BG was still on the property at the time of the shooting.
4.) The BG made no attempt to lay down arms and surrender.
Since the BG threatened the victim with death, the level of confidence that the victim must have before concluding that the BG is retreating and is no longer a threat is very high. How high a level of confidence? Probably somewhere around 99.99%. There must be very little room for error, because the penalties for failure are extremely high; in this case, the penalty is death.
Since the BG made no effort to surrender, and was heavily armed, and loitered on the property even after dropping a jar of quarters, it cannot be concluded with enough confidence in this situation that he was no longer a threat.
If he had left after dropping the quarters, maybe. If he had pitched his weapons and run from the yard, maybe. If he had made himself sufficiently scarce so as to no longer be on the property, maybe. But he didn't.
The victim could have locked himself in a bathroom with his revolver, but that wouldn't have insured his safety. In fact, it may have resulted in his own demise. The BG could have started the home on fire without the victim being aware. Or, the BG could have set up in ambush. Or, simply blasted through the door with the shotgun.
So, all in all, while the victim could have decided that the BG was fleeing, this places the burden of proof, or burden of risk on the victim. The victim has to decide whether the BG is still a threat or not. And, since the confidence in that decision must be high, it is better to err on the side of caution.
The victim did not ask to have that burden placed on his shoulders; his attacker put it there. That's like saying if someone runs a red light at 90 mph, the burden of not being run down lies on the pedestrians shoulders, and that if he should get hit, it's his fault for not getting out of the way.
Let the BG shoulder the burden of possibly getting glock-glock-glocked. He should be the one to do the risk analysis, not the victim. If he decides that the threat of having some caps popped in his buttocks is worth a jar of quarters, let him pay the price for heeding the risks.
On a side note:
One of my buddies once walked downstairs to find a man crawling through his window with a knife in his hand. Him and the intruder were spooked. While my buddy lives in a very liberal city, he believes in home defense. He ran up the stairs and grabbed his Browning Hi-Power. Making his way down the stairs, he checked the lower level for the intruder and found no sign of him. He called the police to report the disturbance. Fifteen minutes after his call, but 20 minutes before the police arrived, someone, presumably the intruder, tried to kick in the side door of the house. Luckily he decided it wasn't worth the effort and left to pursue other interests, but had he gotten in, one can only guess what would happen. The moral of the story is that a criminal's intent is hard to judge. Why take chances?