Clearing up some things...
Self-defense: It wasn't self-defense on the part of the school board security officer, for at one point he wasn't just out of the room, he was out of the building (to retrieve his firearm from his vehicle). Rather, it was both defense of others, as well as the simple performance of his duties.
Missing at that range: It's likely he missed on purpose, as he was trained in the military, had multiple opportunities to hit his target, and was fairly calm throughout, even to the point of reacting to the woman with mild annoyance, nothing more. That's not something something a deranged psycho would do. Most likely his intent was two-fold: Bring attention to the fact his wife was fired, and go out via suicide by cop, not because he was psychotic, but because he was depressed. When that failed, he turned his gun on himself and ended it.
Blanks: No way. Blanks don't blow papers off a desk, leave holes in the wall behind the school board members, or provide the killing blow of the last shot he fired into his own head.
When to legally fire: In this situation, one would have been justified the moment he withdrew his firearm. I personally may have waiting a bit long, until he pointed it at someone, but at that moment I wouldn't have hesitated any longer.
The woman with the purse: Very brave. Very poor idea. If you don't know how to take down a perp, stay away, as it's as likely to do more harm than good.
The 1,000 ft law: This is one of the most ridiculous laws on the books. It's yet another attempt to calm the fears of the anti-gun fanatics while making things even less safe for our children because we can't defend them within 1,000 ft of their public schools. A very similar and equally stupid law is California's UOC law.
Retired cop: I've heard reports he was never a cop, just a trained security guard who had risen over time to be the chief of security for the school system. No solid verification on that, however.
When security became aware of the problem: Immediately, as he was in the room when the guy first took the podium. He first helped people out of the room, ran to his vehicle to get his gun, and arrived back at the door leading into the room about the time the counsel member said, "Please don't." At that point, he opened the door, entered the room, and fired as soon as he could get a shot off.
Ability, Opportunity, Jeopardy / Intent: His shooting skill is immaterial, as MWAG constitutes both Ability and Opportunity. Jeopardy occurs the moment he withdraws his firearm in that context, and the context is all-important, here. Drawing it at a gunsmith's shop for maintenance is an altogether different context. It's intent that's difficult to nail down, but that is in part because it varies the most throughout the states. Castle laws generally imply intent towards an unknown and armed intruder. In a couple of states, however, you almost need a signed confession from the perp, or may have to follow a ludicrous "do not fire until fired upon" rule.
In 20/20 hindsight while watching the video, intent wasn't clear until he slowly drew a bead towards the one board member who said "Please don't," but it could reasonable be inferred intent was stated when he said something about dying that day.
AOJ/I aside, no triplicate form is needed to ascertain that someone who spraypaints a wall then pulls out a firearm, brandishing it menacingly towards others, is a clear and present danger to the lives of self and others. Whether that meets the requirements for a takedown in your own state is something of which each of us need to be certain.