I’m sorry I see how that was confusing. There are two distinct classes of homicides in my mind and i’m sure you’d agree since it was you who first identified 1 of those subsets.
There is one group of gang related these are mostly (all?) with illegal guns. You indicated that this accounts for less than half to half of all homicides. These would probably not be limited by background checks. These are a result of deep cultural norms and societies with values completly different than our own. These (the pereptrators and victims)are generally people who are part of that culture/society with little to no hope of breaking free solving this problem will take a lot more than gun control laws. I dont quite get why they should be excluded from US stats but not foreign stats. again though even if we did you still have to fudge the numbers to make them close.
There is another larger group which includes most other homicides. These would be drasticly reduced by preventing the wrong people from getting their hands on guns. If we enforce are current laws we wouldbe sitting ducks for mentally unstable people who want to gun down innocents. They cant just walk into a gun shop and buy a gun (B”H) but they can take advantage of a gun show sized loophole and buy as many guns as they can carry no questions asked.
Neither of these groups are a result of culture as in TV/movies (which is what I assumed you meant when you first brought up culture way back when)