The reason the murder rates have decreased in the last few decades in this nation is because of new technology in solving crimes and the highest rate of criminals being placed in prisons  ever thus less chance of repeat crimes. This is due to several factors such as "Citizen Demand" to make communities safer and part of that was getting repeat offenders off the streets as well as increasing the police and law enforcement agencies. As a result we imprison more felons than any other civilized nation, a fact that is always in the news as costing this nation billions of dollars annually.  Not all citizens have to have a gun to decrease the murder rates in this nation for sure but those who are strong advocates of arming every citizen with a gun from age 5 to 95 in this nation to "reduce" the murder rate love these stats and show them off often as "proof" to support their ideology. Fact, most guns purchased in the US for "protection" are used on family members.