I'm not sure if being outgunned would necessarily stop the "spree killer" type of crazy person. Most people like that are counting on being killed at the end of their rampage anyway, so fear of their own death isn't necessarily a deterrent .
Another citizen if they were armed, brave and didn't panic probably could have taken him down, but by that time the crazy person has already done their damage.
swedd alluded to my answer. you are correct, you could never ever stop someone from trying to do something like this if they wanted too. and when you put the scenario such that one or two folks in a movie theater/school etc are carrying, they maybe could stop the person from killing 30 unarmed people, but it would require a calm person who can really shoot well in other words it isn't guaranteed at all that they would stop a prepared shooter. but imagine if 40 out of 1000 (4%) of the people in a movie theater, pulled a gun and engaged (not spray and pray, but were prepared to and moved to stop) the shooter? sure, there is alot more risk of "friendly fire" but i guarantee less people would be killed in total and in most instances it would only be the attacker.