It could have been much worse

The shooter at Arapahoe High School (not going to name him, he doesn’t deserve the recognition) came armed for massacre, including extra ammunition, a machete, and Molotov cocktails. It’s fairly obvious from the amount of weaponry carried that the shooter had no intentions of simply killing his debate coach. Few news outlets are reporting on why the attack was cut short, though. It wasn’t because the targeted debate coach had left the school, and it wasn’t because the shooter suddenly had an attack of conscience.
The reason the shooter suddenly stopped his attack and killed himself was because Arapahoe High School had an armed school resource officer who confronted him. The shooter felt he was cornered and killed himself.
Arapahoe County Sheriff Grayson Robinson had this to say:

We believe that the response from the  school resource officer … was absolutely critical to the fact that we did not have additional injury and/or death in this particular matter.

I think we have our proof of concept for armed school officers saving lives.