I can't get enough details from the story to know with certainty that it was a righteous shoot. This is perhaps deliberate on the part of the journalist? I wouldn't be surprised.
I tend to analyze these things on the classic principles of AOJ/I--Ability, Opportunity, Jeopardy/Intent. For new readers, these are hyper-important principles of legally justified self-defense. If you haven't come across them, you must learn them. They are far too important to leave unknown.
I have a problem with the prosecutor's argument that no one was in immediate enough danger to justify lethal force by the guard. Danger is either immediate or not. There is no "immediate enough". Sounds like a fishy way of making something vague so that a door can be opened to acceptance. If the danger is not "immediate enough," then it is not immediate at all, and should be susceptible to being demonstrated as such.
For some reason the judge is saying the security guard didn't have to shoot. And, thought it more likely the decedant was getting into his vehicle, not getting out. Maybe the judge, hearing fully both defense and CA presentations, had access to more data.
I can see a fella getting a gun and shooting in response to the first shot. Maybe even unrighteously firing at their moving vehicle. Maybe even following that vehicle with his muzzle. If these are what happened, I think the security guard shot too soon. I notice that the shotgun was not alleged to be pointed at anybody in the parking lot. The article clearly says it was pointed over the heads...
So, its hard for me to say whether the scecurity guard was justified. It doesn't entirely sound like it, but I cannot say for sure.