I am for capable, responsible citizens being armed. However, it is really difficult to determine who is responsible and capable. (Note: I am very responsible and capable. All of the rest of your are questionable. )
It probably is reasonable to have some tighter checks guns to control who has them. IMO violent convicts and the seriously mentally ill should not have them. It probably is reasonable to force gun owners to have a way to lock them up.
By removing all guns from society you probably could reduce the rate of murders. But would you increase the assaults, rapes, etc.? A gun is an equalizer between petite 90 pound woman and 350 pound ex-NFL lineman.
This isn't a simple choice.
IMO if someone is really hellbent on creating a massacre, there are many ways to do so. Firebombing, pipe-bomb etc. are fairly easy to make. Or for that matter a knife or sword is a very deadly weapon. Or a bow or crossbow for that matter.