OK, I'm going to throw out a lot of ifs here...
IF I knew he would still be a danger after his attack and
IF I could safely (behind cover) get within 50 yards of the entrance to ambush him and
IF I could have my pistol on a rest and
IF I thought I could get a safe shot (safe behind the target) and
IF I could identify the shoot before he walks out (I don't want to shoot someone that subdued him and then took his weapon) and
IF I thought I could stop him before he had time to realize I was even there (basically: can I tell whether or not he's wearing body armor. I'm confident I could make a center mass shot at 50 yards, but not so confident about a head shot)...
I might think about going across the street and taking this guy out. I mean, I'm not the kind to say that we need to go vigilante and use our permits as police badges. But this was an obviously very dangerous situation-I'm not going to mistake full auto AK fire for a self defense situation or anything else, this guy was obviously out for blood. And how am I supposed to know he's not going to come across the street and shoot up my place?
I would definitely feel my life is in danger and want to stop the threat while I still have the upper hand (I can't remember about NC, but we have no duty to retreat in TN). This is one of those situations where you'll have to live with whatever decision you make for the rest of your life. After this guy walks out of the building, he shoots some chick in the head trying to get away on a motorcycle. Granted, she didn't die (miraculously), but if I knew I could have prevented more deaths outside of the building and didn't, it would be hard for me to live with that decision.
I know to not go looking for or running towards danger. But in this situation I haven't found danger, danger has found me and it's right across the street. I have no duty to retreat (not in TN and not in AZ) and I'm not storming into the building, but I would feel my life is in danger with a crazed gunman in a building across the street. Putting myself in a safe position and shooting to stop the threat seems like a reasonable course of action to me.
A couple more things: I'd have someone else call 911 or do it myself before hand if I'm alone. Also, if this guy had a pistol, it would change things greatly. On the one hand, I wouldn't feel so out-gunned and have an easier time facing him off. But at the same time, a pistol doesn't have the reach his rifle does, so I don't think I'd feel like I was in as much danger across the street. That may cause me to want to stay where I'm at and wait for the cavalry, protecting myself if he came towards me.
Honestly, I think the decision is easier knowing this guy had a full auto rifle, capable of firing across the street and down near the next neighborhood. Across the street I'm in the kill zone. With a pistol, not so much.