If you're paid to be an armed security officer, I would assume that there is an expectation for you to actually act in a situation that would require the help of an armed security officer.
It's not like anyone expected the guy to get in an OK Corral one-on-one shootout with the kid. Anyone who has a clue how police tactics work would never do that in the first place. When you have a target who is being anything but stealthy and not maintaining a defensive position, it's easier to get the drop on him. Alternatively, at the very least, making it known that there are guys with guns in pursuit of him could put a damper on a wanton killing spree.
My question was more about the normal procedures in a situation like this and not what people expect. Cops here a taught to wait for reinforcements before taking action, the reaction from the Sheriff made it look like they are supposed to jump in without thinking. There's a lot of information he couldn't have at that time, like where the shooter is located and or even if there are multiple shooters.