This excerpt from the Cornered Cat, pretty much sums it up for most cases of self defense or defense of others:
Awhile back, some lowlife in Portland, Oregon, kidnapped a 10-year-old boy. A witness reported the kidnapping, and gave a description of the vehicle along with the license plate numbers. Later in the day, someone who’d heard the account on the news spotted the pickup truck as it turned onto a dirt road outside of town. The police converged on the scene. When they arrived, the man was standing near the boy, waving a firearm around. When the kidnapper turned his gun toward the officers and began shooting, the officers returned fire, killing him. The boy was unharmed (except, of course, for psychological trauma).
Did the criminal deserve to die? Well, whether or not you believe he deserved to die for kidnapping a child, the fact is that when he raised his gun to fire at the officers, the criminal had just made the most important choice of the day. He decided that someone was going to die.
The officers on the scene had to decide whether the person who died would be an innocent child, or one of themselves, or the criminal. The choice that someone was going to die had already been made, and the good guys weren’t the ones who made that choice.
There is some excellent thinking on The Cornered Cat. Recommended.