So if everyone owns guns, then murder rates might go down.... Great, USA jails must be nearly empty....oh, wait a sec...! 
Read the article. An interesting point is that repeat offenders are less likely to engage in direct confrontational violence in places where the victims may be armed with guns.
Another is the inverse relationship of gun control laws and overall level of violent crimes between states in the USA. There's also quite a bit about the growing violence levels within the UK as gun laws get tighter and tighter (ironically, with the intention of reducing violence).
Since most violent crimes are the result of repeat offenders, those over-filled US jails are at least partly responsible for the levels of violent crime in the US (including homicide by gun) slowly falling off. Personally, once elected as the Emperor of Earth, I'm going to rearrange sentencing to limit the punishment for victimless crimes to nothing more than a fine (or, in some cases, a spanking), but plan to keep repeat violent offenders incarcerated. They'll be hard at work for many years, forever running in giant hamster wheels to generate electricity.