He is armed with a knife, is already repeatedly stabbing her, leaving himself wide open for a shot, and I am armed with a firearm...That's obviously entirely different, and in a situation such as that, who wouldn't act if armed? But standing in line at the quickymart, and someone is demanding cash, I am not getting involved...
Armed or not, family or not, I am all over that guy. Thats just me. She could have murdered his children, or she could be your Mom or Wife. If you hesitate she is dead.