One of the most important effects of a few people carrying concealed in an area (or even just the permission being given to do so) is that it makes the potential attacker hesitate. He knows that everyone is probably not carrying . . . but he doesn't know who is carrying. That element of uncertainty could keep anything from happening in the first place.
It certainly works to prevent "hot" burglaries in the South, as opposed to D.C.
And, of course, if somebody does have a weapon and either happens to be in the building or "rides to the sound of the guns" as students did in the Texas Tower shootings, then the attacker is pinned down, deterred, or eliminated before he can add to his list of victims.
Not true based on the facts as we know them. Based on the info we have it appears we have a suicidal person with extreme rage wishing to vent destruction and having no concern about his own life. That is evidenced by him taking his own life rather than attempting to flee. In this case gun control legislation or lack of it was irrelevant to the outcome.