Microcosm: Georgia and Kansas places require appropriately able people to have guns.
Macro: Let's suppose (for argument's sake [we all love a good argument, right]) that this became federal law. I am thinking about doing a crime anywhere in the U.S. I am shit scared, because chances are folks are packin' heat. Or, I take the risk, knowing there's a good chance I'll be drawn upon. The overall crime rate goes down. Great!
Fear is a great motivator.
But we all start becoming fearful of our neighbor, not knowing the secret lives of others. Maybe that argument you hear next door becomes much more significant when you know there's two pieces in the house (husband and wife each required to have a gun).
So the crime rate goes down further, cuz you don't want to report that domestic. Hell, what if he/she finds out you made the call? FEAR.
BUT , the homocide rate goes up, I'll bet, even though the overall crime rate may go down.... for a while...
The repression eats away, and you have your own weapon, and you're paranoid now...
Wait, I had some entropy to go allow - bye.