Let's suppose there are two countries, A and B, which in terms of social development, education, prosperity and crime control are comparable.
In both of them, the chance of homicide from all methods is one in 100,000 per year.
In country A, where guns are rare, the chances of homicide by firearm are 0.06, and in country B, where guns are less rare, is 0.21.
But since the total homicide rate is the same, you might reasonably say that potential murderers have simply chosen to use a gun because it was easy.
Now consider country C, where guns are very common and the chances of homicide in total, are 3.8 in a hundred thousand per year, and of homicide with a gun are 3.43.
In country C, you might reasonably say that some of the potential murderers have simply taken a gun as their weapon of choice, but the ready availability of guns is not only associated with an increase in the number of firearm homicides, but also an increase in the number of homicides. Whether this is because an attack with a gun is more likely to be fatal than an attack with a hammer or a knife, or because it is psychologically and physically easier to kill someone with a gun, we cannot tell.