OK- His mistake caused 11 percent of the casualties. Let's say he did NOTHING, and the perp figured out the safety catch on their weapon. Now how many casualties are there? That's the same Catch-22 of because the person stops a mass shooting from occuring that there wasn't a mass shooting.
Would you have the same derogatory opinion if the shot(s?) at the Deputy and the wounded student were fired by another responding LEO, or would that be just be an unintentional "fog of war" result of a high stress situation?
Would any of this help the guard when if/when the DA decides to throw the book at him? Probably not, but the prosecution sure wouldn't want me on the jury.