If you can not prevent a sick person to improperly use gun does not make the rights to bare gun wrong. If one want to argue when a gun crime happen, where are those legal weapons in defense? That is right, because the gun regulation, citizen normally do not carry gun everywhere pretending there is danger in their normal life.
Let me adopt your logic to the 9/11 case. Terrorists use air planes to attacked our buildings, where are our fighter jets that we rely on to protect us? So, in your logic the USA should not have Air Force?
We have to learn to unexpected circumstances differently.
For those not getting how USA citizen could defend them from the over power government, think if the China style of knock down building could have happened in the USA. Citizen here would shoot the hell of anyone want to get in their property without their permission.