I just commented in schneier's blog so I may as well reproduce it here. In response to this comment:
I remember a debate after the Virginia Tech shooting, basically the 'if everyone was armed this wouldn't have happened" argument.
Both the tripod and mp3 cases are prime examples of why allowing the general population to run around armed is a bad idea.
Over and over it's been shown that people misjudge what they see, make bad assumptions, jump to false conclusions, and react inappropriately.
Look how bad it is now. Imagine how badly these cases would have turned out if anyone of the helpful bystanders involved had pulled out a gun.
You have this 100% backwards.
Ordinary individuals have extremely powerful incentives not to shoot people on mere suspicion. The prospect of going to jail for the rest of one's life tends to encourage armed citizens to double-check whether that's a gun or an MP3 player.
The police? We know of many cases where people have been murdered by the police for wielding a wallet, for looking too much like the wrong guy, for failing to follow instructions quickly enough, for defending their own home against armed intruders who turn out to be cops at the wrong house, and too many other variants to count. What have the consequences been for those police? They have varied from the typical "absolutely nothing" to "light reprimand" to, at the absolute worst, getting fired.
Who is more likely to pull the trigger too fast, the man who is at worst risking his job or the man who is definitely risking his freedom? But you don't have to take my word for it: how many cases do you know where an armed citizen shot an innocent because they saw danger where there was none? You know that at least tens of thousands of ordinary people carry concealed firearms, right? Where is the carnage that you presume must result?
In the US, when someone wielding an MP3 player in a public place is taken down by a SWAT team, more than likely several armed individuals had a chance to draw on him and somehow managed to restrain themselves. The police of course could not since they treat every such situation as one threatening imminent violence.
An armed populace can respond to a violent altercation much, much faster than the police ever can, are much more likely to witness the precipitating events directly, and have everything to lose when pulling the trigger. The police come into a situation late, cold, and with get-out-of-jail free cards for all the officers should they happen to murder an innocent. I know who I would bet on to make fewer mistakes, and it's not even close.