I think this case is still taught in the "righteous shoot" portion of police academy training. I tell it as I got it from a deputy sheriff.
About 20 years ago, three freelance socialists were buzzed into a jewelry story that dealt mostly in diamond jewelry in the Greater NYC area. They pulled pistols and threatened the customers, clerks, and the owner of the shop. The owner had a pistol and a pistol permit, probably a premises permit given the location. In any case, he got his pistol and commenced firing at the perps, dropping one of them as the perps returned fire. They decided things were too hot for them, and bolted out the front door and ran down the sidewalk. The shop owner followed them onto the sidewalk and dropped both of the fleeing perps.
The people in the area were appalled when the jeweler was arrested, tried, and convicted of murder. By street standards, the shop owner was just defending himself, his customers, and his property.
But if you break the incident down, the verdict that sent the shop owner up the river is the correct one. There are two parts to it, a point missed by the common folk.
When the perps pulled guns and threatened people in the shop, the jeweler was justified in defending himself, the people in the shop, and his property. His shooting of the goblin who was shooting at him is a righteous shoot. There was an imminent threat and he had every reason to be in fear of his life.
But when the other two perps fled, the imminent threat ended and so did the fear of life. The moment he followed the perps out of the shop, HE became the aggressor. He shot and killed the two perps who had tried to rob his shop when they were not threatening him or anyone else. That makes him the guilty party, and that is why he was convicted of two counts of murder.
There was no threat here, and that ex-cop certainly knew better. He just thought he would get away with it because he expected a certain amount of professional courtesy from the serving officers. He did not get, and did not deserve, that professional courtesy. At that point, he was merely a civilian, got the same treatment any other private citizen would get.