It has been proven repeatedly that bans do not make spaces safer from those with criminal intent. Violent criminals and terrorists do not obey these bans...only the generally law abiding who are harmless. All they do is create a potential for useless charges against someone who means no harm while serving absolutely no useful purpose against the malevolent intent found among active shooters and terrorists. Dooming everyone who tries to live their life playing by the rules to hope the police show up quickly enough to prevent them from being killed by the sorts of malevolent actors causing the carnage is ridiculous and counter-productive. 

If the goal here is the preservation of innocent life, there is abundant evidence that loss of life in an active shooter scenario is limited only by how long it takes for armed resistance to appear. People dial 911 hoping the police will arrive and turn an intended slaughter into a gunfight. Having an armed citizen among the intended victims accomplishes this goal much sooner and limits the damage until police can arrive to take further action if it is still necessary.