You're statistics are somewhat biased, seeing as many of the numbers are provided by a gun control organization.
Also, I think you need to eliminate the suicides, as they will likely have resorted to other means if a gun wasn't available. Their inclusion really muddies the waters
What you really want to check, I believe, is per capita homicide rates by country, and see if this is at all in line with number of firearms. Having said that, I still stand by my assertion that factors beyond simple firearm numbers account for homicide rates.
I checked
another wikipedia page listing homicides per capita worldwide and the USA is quite far down the list (ranked 108 globally), at about 4.2 per captia. Canada is a further down the list, at 1.6.
At the top of the list are some horrific numbers with the top 20, ranging from 26 to 91 homicides per capita