Another vegetarian NRA member here. I'm with Dust In the Wind; you have to frame your position in terms of family security.
The wife seems unwilling or unable to discern between killing and murder. The argument might go something like this:
if a bad actor ever tried to kill your child, SHE is responsible for saving the child's life. If she does not do everything possible to stop the BG, including killing him, then she is, in effect culpable of murdering your child.
If she has the capacity to save her life by killing the BG, but doesn't, she is guilty of her own murder, possibly depriving you and your kid of a wife and mother.
If she could save YOUR life by killing a bad guy, but lets him go, she is responsible for your murder.
It is a sad fact that criminals sometimes force us into these situations where we must take one life in order to save others. In this case, any response, action or non-action will result in death. Nature dictates that we preserve our own lives and those of our families.
She has "evolved" to the point that she has lost her very instinct for self preservation. Sad that a small mouse would have the sense to fight for its life against a threat, even a giant human, but this woman is "past that".
Of course you'll want to use euphemisms as you will be punished for opening her eyes to the real world. I'm not invested enough to think of nicer ways to say these things.
Avoid the Joe Horn argument. That is a red herring. You don't HAVE to get involved in a criminal attack against someone else. That is asking way too much from her, but expecting her to defend your family is not.
ANOTHER TACK could be asking her help to educate your son about the history of the US, the ideals of the founding fathers, the principles of freedom. Hopefully she will learn to at least respect the RKBA, even if she personally would not exercise this right. Whew! Sorry for the long post!