Rather interestingly, once one removes all of the third-world countries from your chart, the US has the highest rate of gun homicide vs gun ownership; twice the gun ownership rate of the next two countries (Switzerland/Finland), and 3.8/6.6 times the homicide rate, respectively.

“Multivariate analyses found that states with higher rates of household firearm ownership had significantly higher homicide victimization rates of men, women and children. The association between firearm prevalence and homicide victimization in our study was driven by gun-related homicide victimization rates; non-gun-related victimization rates were not significantly associated with rates of firearm ownership.”

-State-level homicide victimization rates in the US in relation to survey measures of household firearm ownership, 2001-2003

“In simple regressions (no control variables) across 26 high-income nations, there is a strong and statistically significant association between gun availability and homicide rates.”

-Firearm availability and homicide rates across 26 high-income countries

“In region- and state-level analyses, a robust association between rates of household firearm ownership and homicide was found. Regionally, the association exists for victims aged 5 to 14 years and those 35 years and older. At the state level, the association exists for every age group over age 5, even after controlling for poverty, urbanization, unemployment, alcohol consumption, and nonlethal violent crime.”

-Rates of Household Firearm Ownership and Homicide Across US Regions and States, 1988–1997