Correlation (Voting Republican and poverty best predictors of state murder rate)
...While the causes of individual acts of mass violence always differ, our analysis shows fatal gun violence is less likely to occur in richer states with more post-industrial knowledge economies, higher levels of college graduates, and tighter gun laws. Factors like drug use, stress levels, and mental illness are much less significant than might be assumed.
http://www.theatlantic.com/national/archive/2011/01/the-geography-of-gun-deaths/69354/
After voting Republican, the biggest correlation is poverty, which it is with all social ills.
Two correlations that are not included...
Obesity probably doesn't cause gun murder, but the state-by-state correlation with obesity is much higher than the correlation with drug use or mental illness.
Alcohol use, unlike obesity, is directly causative and should surely have been included. Alcohol use is probably a high predictor of any violent crime.