I've tried to see all sides of the failure of Scot Peterson to enter the high school on Valentines day. Playing the devil's advocate for a while brings up at least some points worth considering on what really comprises a "good guy with a gun".
First, I would say tactical/strategical.
To defend against any high powered weapon body armor under and including 3A is insufficient. Hard plates and helmets are a minimum. The deputy - to the best of my knowledge - was not equipped.
As well, an equivalent weapon is needed to engage. We often joke about those that bring a knife to a gun fight but bringing a 9mm to a .223 fight is almost as futile.
Next we have psychological.
In hindsight, maybe a 54 year old making at least $80k a year and can soon retire with 75% of those wages and benefits isn't the ideal 'nothing to lose' candidate for the front line of deadly incident.
In the same breath - physical condition.
None of the biathalon skiers in the Olympics have bodies like Peterson's. It takes great fitness to move rapidly, find concealment and cover (with the heavy gear and arms outlined above) and then to be effective and accurate in shooting. The deputy would have been considerably out of breath after running 30 yards and up one flight of stairs.
Numbers.
While it might be known in real time this was a single shooter incident, in the fog of war there is often real confusion and doubt. Columbine had two armed shooters. The incident in Beslan a few years ago featured 'several dozen' armed attackers. 1 on 1 doesn't give the defender or the students the odds they deserve.
My initial thought about Scot Peterson was that he failed those kids and failed his duty. He should have been at least in place to take advantage of a clip reload. You wait for your opportunity and then take your shot.
He should have at least been trained enough and capable of that. Maybe he never had it, maybe he reached his breaking point, who knows? But we do know this, several other 1st responders (Broward County) hesitated/failed that day also and are now on leave and review.
This is further complicated because police were watching surveillance video that was 20 minutes delayed and believed the shooting was still live for some time after it had stopped. Bad intel never helps.
https://www.policeone.com/active-shoote ... -shooting/
All of the above combined lends one to see this more as a systemic error.
Sheriff Israel and anyone that saw Peterson should have sussed him out to be a kind of 'good hearted schlub'. Maybe good at high fiving the kids in the hallway and knowing where the donuts were in the teachers lounge but NOT the guy to have the tools, skills, psychology and conditioning to effectively counter an armed attack with a high powered rifle.
Like someone else wrote here "he wasn't a good guy with a gun, he was just a guy with a gun".
If school guards are going to be effective they need all of the essential elements stacked in their favor: the right tools, the right skills, the right psychology, and the right conditioning along with the numbers to proceed without hesitation against 1 or more attackers.