It's important not to forget the people who would have died if they didn't have a gun to defend themselves, even if your coworker would have had a better chance if guns were banned. Of course we don't cry for the people who would have died if guns were banned, but they are just as important as the people who do die. Even studies carried out by anti-gun researchers show guns are used in self-defense hundreds of thousands of times a year in the US. Though of course the large majority of those defensive uses didn't prevent deaths, a significant number surely did. One of the saddest things about gun crime is that the victims who could benefit the most from arming themselves are women, who mostly voluntarily go unprotected. It's not fair to disarm the few women and others who do want to protect themselves, in order to protect those who chose to be vulnerable.
Even if a gun ban would prevent more street murders than legal guns do, one of the countries you cite, China, is a perfect example of how if the people were armed, millions of deaths that result from the poverty caused by the tyranny of their murderous criminal government could be prevented. I think the people of North Korea would gladly accept the street crime murder rate of the US if they could get rifles to help overthrow their government.