Wrong. You have a hypothetical scenario. Do you actually believe that this type of mass murder can't or won't happen. Wake up. What is reality is that I have the ability to protect myself if I'm ever in a place where this is happening. You obviously don't have the same ability. You can either be the sheep or the Shepard. It's obvious which one I am and which one you are.
EDIT: I apologize to the other people who have posted on this thread. The main focus should be on those who were killed, injured, and the family and friends of the aforementioned.
Identifying the threat is the major issue IMO. I was at a shopping mall before this shooting in Colorado happened. The mall was under construction and loud synchronized noises were coming from the hall. Everyone just went about their business like nothing was wrong, which it wasn't. But who's to say that a nut case wasn't up the hall with a gun?
The amount of chaos that breaks put when something like this happens must confuse all the senses and by the time someone gets a firm grip on their surrounding and what's going on its most likely too late even if they are equipped. This is where law enforcement really shines through and you have to think about how tough it must be for them to always be "ON".