The first thing you have to remember Jake, is that regardless of how unfortunate and sad this particular event was . . . . if you were a law abiding citizen with a CCW, you would probably not be allowed to carry a concealed weapon into the mall or the theater. To the best of my knowledge, this is pretty much universal in each state.
Second, you have to consider the chaos of what occurred. Regardless of how good a shot you are (or think you are) or how much practice you have had or training, it happens so fast that you do not know how you will react. In that situation, just think of the number of innocent people that could have been injured or killed if some good intended person, carrying a weapon (whether it is leagal or not) decided to return fire.
I'm not being a damp rag on your question as it is a valid one. I think you also have to consider the possibilities of what could have occurred if someone decided to return fire. Had the LE been present, even they could have had problems with all of their training.
I was up in Tucson the day of the shootings. Several of us were on our way to a range to shoot. That pretty much made things quiet and sober in the entire city when the news got out. AZ has some very liberal gun laws and there are many who carry - myself included. As we ate a late lunch that day, we were discussing the facts as we knew them at the time and wondering why, with as many people who carry, someone hadn't taken the shooter out. Let's face it . . when these things happen, they happen fast . . . and I would imagine that even a well trained LE would have a delay until the situation "set in". That's not a criticism of any LE officer . . . it's just that it is not an immediate reaction response to what is happening. Most times when these things happen, the perp deliberately picks a crowded situation . . . and of course with that scenario, there is always a greater danger of an innocent bystander being inured or killed by a well intentioned person who thinks they are doing the right thing.
As you get further into shooting, as already mentioned, the key is to practice, practice and then practice more and get as much training as you can. Know your weapon, its capabilities . . . and know your own capabilities. It's not about the bullet, it's about placement.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63