can be 'screened' for mental illnesses that would cause them to commit such crimes...the only way to dramatically reduce the likelihood of gun homicides and mass murders is to dramatically reduce the number of guns available so that such people can't easily obtain them.
Most importantly, the general populace does NOT NEED guns...they are for "Pleasure"...i.e. Hunting (you can get meat at Safeway), but allowing hunting rifles with today's controls is not high on the list of concerns...Target Shooting...use a gun range's AR-15, etc. if you want to shoot such weapons...not unlike going to a bowling alley for fun.
As for the other claims of "Need"...e.g. Home Protection...evidence shows that there are far more accidental and domestic dispute gun deaths each year than thwarted home invasions with a gun...not to mention suicides from firearms.
Finally, as regards defense against "Tyranny"...we have a well established process for resolving disputes about government policy...i.e. the Voting Booth. Anyone who thinks they could prevail against a professional military with all of its weapons is literally insane.
Fortunately, we have recent examples of other countries effectively reducing the number of guns in the population...Australia, New Zealand and Canada have instituted "Buy Back" programs as well as bans on military firearms, including ammunition...and they've worked, without a mass uprising of the populace...we need to do this.