An armed citizen would have been likely to add to the carnage IMO. Read up on this, and these are trained, active duty police officers.
http://www.cnn.com/2012/08/25/justic...ing/index.html
And how many people have been under fire in Vietnam etc compared to the number of gun owners? Maybe one or two percent? About 20 million vets, say 10% ever under fire--which might be high, so yeah, one or two percent tops.
You insist on concentrating on the exception, not the rule. The average person/gun owner does not have the proper training for this kind of circumstance. Think of the word "average"--got it? The median, the norm, not the exception.