She's pretty much on the mark with the reason for the existance of guns these days. They're almost exclusively designed to kill.
I have no problem with acknowledging that fact. I don't think you should either.
I submit taking the wind out of her sails, and pointing out that most people believe there's nothing wrong with killing as long as the right people are killed. (As evidenced by arming of police, military etc) People who threaten my life, for example. Killing animals for food is also quite acceptable.
If killing under any and all circumstances is unconscionable to her, including if she's about to be raped or her family stabbed or something, (It's possible, some people really are that idealistic about life), try the competition angle: Just like archery, darts, pool, snooker, etc, it's a sport which involves use of skill to achieve a measurable goal.
NTM