I've seen this discussed elsewhere, and I've seen a short video with still photos of the event in question.
It's really an ugly event.
I won't use terms like "security guard" since there's statements he wasn't a legit guard in that circumstance. Pinkerton denied he was one of theirs.
So, we have "shooter" and "shootee", for lack of a better term.
In the clip I've seen shooter grabs at the shootee for some reason. Shootee slaps him and backs away. Shooter draws as shootee backs away; shooter does not back up himself. Shooter appears to shoot AS the shootee is backing away and spraying;
there is a still image where you see the beginning of the spray cloud, AND you can see the empty shell in the air and the slide still out of battery. In other words, shooter had already pulled the trigger before being sprayed.
So I'm sorry, I can't see any justification of this. One guy is holding his can of spray, so you know what he has. He raises it as he backs away from you, which is a defensive posturing, not an assault. You don't get to shoot him for just that.
As the guy isn't licensed to be out there, he doesn't need to be trying to take the spray away from someone else, or otherwise messing with somebody. That's a matter for police or real security. You leave him alone, and if he sprays you the police will attend to him. You don't get to shoot him for finding him moderately offensive