And also completely wrong!
I'll admit that it was a momentarily amusing thought with no solid base, and that there are obviously a thousand other factors involved, but you're actually the one that's completely wrong.
If the US has the highest murder rate (which is the assumption you are basing this "proportion" on), then all countries with lower (or zero) execution rates (ie. the overwhelming majority (>90%) of countries in the world) would have lower murder rates.
Are you suggesting that they all have... higher murder rates instead? Does Slashdot not understand basic mathematical relationships anymore?
Inversely proportional means one stat goes up as the other goes down. The US has a higher homicide rate and a lower execution rate than the other four countries, therefore the relationship between homicide rate and execution rate is inversely proportional. This place used to have math nerds...