In any specific situation there is no telling what will or won't happen. In the Oregon shopping mall incident as soon as a concealed-carry permit holder drew his weapon (didn't fire because of too many bystanders), the attacker fled into a stairwell and committed suicide. Who knows how many lives were saved. In the New Life chapel attack, a security guard engaged the gunman and who knows how many lives were saved.There was an armed guard, but bringing a pistol to a rifle fight just doesn't work.
Do we even know whether the armed guard was anywhere near where this all went down and, if they were, were they in any position to actually engage the shooter. The shooter apparently cased the place pretty thoroughly and I would not be at all surprised if that included choosing the time and method of attack specifically so as to mitigate the ability of the sole security guard from being able to react effectively within the planned duration of the attack.