Guns kill people and other guns.
If mass murder results in the restriction of weapons ownership, then guns kill other guns. Or, you could argue that guns don’t kill guns, people kill guns. Or, that chemical reactions in the bullet kill people. Chemistry kills guns. Or that evolution, which demands of higher intelligence the use of spatial/temporal references, kills guns eventually. After all, it took millions of years for humans to develop the ability to shoot things dead. Guns are not part of any unified theory. Warfare demands innovation. Death kills guns. But we’re stardust. Gravity kills people. Gamma radiation kills Hot Pockets.