I’m so sorry, but a gun by itself cannot kill anyone. We don’t need gun control, we need NUT control.
You've swerved into something here. Most 'nuts' will look for a place to act-out where they are least exposed to the risk of being taken down quickly. That's why they target schools, malls, churches, etc. rather than walking into a police station, for example.
Armed citizens (as proposed in the 2A) would serve as a deterent the same as thousands of undercover police would. This effect wouldn't be limited to just the crazed psycho killer- it would deter criminals of all types.