You might start with the idea that in a defensive shooting scenario, the goal isn't to kill the aggressor, it's to stop the threat. I've been successful many times using this argument against the, "handguns only have one purpose: to kill." crowd.
I can also have my pistol ready for action to defend myself and my family a lot quicker than they can dial 911 on their cellphone. And I don't have to wait 5 minutes or more for my pistol to arrive. (Not a slam at cops. They can't be everywhere when needed, that's just a fact.)
When criminals know that their intended victim may be armed (because they live in a shall-issue state), they tend to shift their activities to crimes that require less "interpersonal" contact. And just the possibility of an armed victim can cause this shift. No shots need be fired. Two lives are saved: the intended victim, and the slimebag criminal.
That's just a couple of ideas. I'm sure you'll get more.
Good luck.