Protestations of gun rights supporters aside, public health researchers who study firearms generally agree that increased firearm ownership rates are associated with higher rates of homicide.
A a 2007 study by Harvard School of Public Health researchers, showed the correlation after controlling for robbery rates:
The Harvard School of Public Health’s Injury Control Research Center is a great resource here. It notes that a wide variety of methodologies show guns as a risk factor for homicide in the US and other high-income countries. Developed countries with more guns generally have more homicide; states within the US with more guns have more homicide; people with access to guns — particularly women — are more likely to be victims of homicide than those without access.
It’s important to note, however, that all these studies show an association, rather than causation. It could be that areas with more guns are more prone to murder for other reasons. But the fact that the finding holds up no matter how you approach it is suggestive, and most experts think the relationship is at least partially causal. ”Within the United States, a wide array of empirical evidence indicates that more guns in a community leads to more homicide,” David Hemenway, the Harvard Injury Control Research Center’s director, wrote in his book Private Guns, Public Health.