If every person had a gun on them at all times, you would have far less VTech/columbine type incidents, but far more isolated shooting incidents. It elevates every confrontation to a potentially life-ending situation, and also greatly increases the risk of accidental gun deaths. Do we want people to have guns on them while they're drinking 20 beers in a bar and looking for a fight? Do we want guns around children all the time? Do we want to sit in class surrounded by guns? I actually think in this one instance, lax gun laws contributed to the mass killing, because it would not have been particularly easy for a foreign loner with personality and social disorders to find guns and ammo on the black market in a small college town.