One would think it's logical to carry guns in case someone goes crazy and starts shooting and then someone will be able to stop them.
But let's think about it for a second.
First off, shooters are typically your every day man. Basically, what it means is that you won't be able to tell if a person is aggressive or not based on his appearance. In the most recent attack by Cho, I believe he wore a vest, which might tell him apart, but wearing a vest if pretty common.
Second off, let's suppose three people got out their guns in hopes to stop shooters. Let's suppose each of the three don't know each other. Now, how will one tell apart from the other? Let's say one starts shooting the actual shooter, then another sees him, and thinks he's the shooter and starts shooting at him? Innocent person got injured for no reason.
Third, factor in fear. The sound of gun fire or any loud sound is known to cause people to shake. It's a natural reaction since it's an indication of a problem... in nature it could mean something is going to attack you, an earthquake, things falling around you, etc. With that in mind, you'd be shaking like a broken motel bed add that in with the above two and you'd get crazy people shooting at things wildly.
In my opinion, I think schools should educate people at a young age on how to cope with various life problems. You can't tell people to be nice to everyone, but you can tech them how to cope with problems in a safe way.
I am against any idea to arm citizens in hopes to prevent further problems. I think the average man is too dumb to use it effectively outside of his home. Shooting targets at a range is easy, but try shooting a target in a crowded area while the target is moving around things you don't want to hit.
In Texas during the 70s, the reason they invented SWAT teams was out of this dude who decided to buy all these guns and take control of a rooftop of a university where he killed like 10 people or so. The reaction of the public was to bring out their own rifles and fire back. Needless to say, the confusion caused the police to react to the local people shooting back, and on top of that, finding a way to break down the door to the rooftop where the man has like 1000s of rounds and 4, or 5 different weapons. The end result is the invention of the SWAT team, but the more important thing is that weapons used by civilians to counter defend is a bad idea and not ideal as more people got killed and injured as a result of the gun fight.