The murder rate is declining, mostly because the population is aging. The number of massacres, however, is rising.
You can speculate about some faraway place in the Land of Nevernever where guns are like magic wands and we all ride pink flying unicorns, but the more guns there are, the more likely it is that some crazed nutcase, like the Sandy Hill guy will grab a gun and go a-hunting.
I don't think that a lot can come of legislation that will stop the massacres, but I do think we could do more. The idea that somehow gun nuts owning more and more guns will make us safer is ridiculous. Eventually, the gun nut dies and the guns are still quite deadly and could end up in the hands of mass murderers.
When several of the dolts on my block get drunk on July 4th and New Years, they think it is fun to go outside and shoot off their guns. If there were fewer guns and/or ammo were expensive, they would not do this.