"This data set provides a multinational example of the central point that lethal violence is the crucial problem in the United States," Zimring and Hawkins write. "It shows the United States clustered with other industrial countries in crime rate, but head and shoulders above the rest in violent death."
Why does this happen? It's not because, as you might think, American violent criminals are just more likely to kill people. "Only a minority of Los Angeles homicides grow out of criminal encounters like robbery and rape," they find (there's no reason to believe the pattern would differ in other cities). So even if it could be shown that American robbery and rape rates are across-the-board higher than those in similar countries (which
doesn't appear true today), that still wouldn't explain why America has so many more homicides than other countries.
Again, Zimring and Hawkins's LA data was revealing. "A far greater proportion of Los Angeles homicides grow out of arguments and other social encounters between acquaintances [than robbery or rape]," they find.
This is where guns enter the story. The mere presence of firearms, according to Zimring and Hawkins, makes a merely tense situation more likely to turn deadly. When a gang member argues with another gang member, or a robber sticks up a liquor store, there's always a risk that the situation can escalate to some kind of violence. But when people have a handheld tool that is specially engineered for violently killing, escalation to murder becomes much, much more likely.