Your assumption that a "gun death" is somehow worse than any other unjustified homicide is worse is not correct.  And to get your stats to work out in general, you need to very carefully cherry pick the countries that you consider "DEVELOPED".  Also, you have to consider that many European countries only count a death as murder after the killer has been caught and found guilty.  So, if they have a 25% rate on solving such crimes, their numbers would be 4 times larger when compared to the manner that the US reports murders.
Maybe if we compare our crime rates and gun laws to Mexico our closest neighbor we could get a more accurate picture of how gun laws affect crime.