I remember at the time of the Ecole Polytechnique shooting (14 dead) my mom (who worked there, but was out of the building at the time) was puzzled as to how nobody tried to stop the guy. Strangely enough, I don't recall my dad saying anything similar (he's a prof there, and I think he might have been in the building at the time).
When I read through the detailed story of what happened on wiki a few days ago (
http://en.wikipedia.org/wiki/%C3%89c...ique_massacre: note: this is a detailed account so it's chilling to say the least) it does seem like someone could have maybe stopped him (he went through 3 different floors and his gun failed to fire at one point, so he needed to reload). However, I do realize that thinking someone could have stopped him is clouded by the fact that I'm looking at the story as an outsider, after it happened, so I have all the details at my disposal.
The reality is, when crazy events like this happen, I can assume the people that are caught in the crossfire have no details about what is going on other than the fact there's a guy unloading a gun. They don't know his full intentions, how many rounds he has left, where he's headed next etc... I can also imagine that actions and responses are clouded by fear and the natural human response is to flee...
Regular civilians have no training to deal with events like these or to "stop a gunman"
That's where police training comes in to enable cops to deal with gunmen. Aside from the fact they are armed and where protection, cops are trained to deal with the stress and evaluate the details of the situation properly. They are also highly trained in the use of their weapons.
It's very disturbing to me that many people seem to think that arming civilians is the answer to prevent this kind of event. It goes to show the cultural gap in terms of guns is huge between Canada and the US, because I don't recall many people thinking that if others were armed, the massacre at Poly could have been prevented. I walked passed the memorial of the 14 women everyday for over 2 years, but I never once thought that if others were armed, they would still be alive...