Everyday there are examples of armed citizens using firearms to defend life and property, often not their own.
In Texas, a great gun state, a woman was confronted by her husband outside of the school where their son attends. In the midst of an argument, the man pulled a knife and stabbed his wife several times. A bystander drew his legally carried handgun and stopped the attack and held the man for the authorities.
While the woman is in critical condition, had the bystander not been carrying that day, she would probably be dead, and others could have been hurt.