Re: The reports so far with some editorial....
I have not decided (though I live in the UK where firearms for defence are not allowed so this is a little hypothetical) before hand that I will kill people with my firearms. I have decided that if I had to I would.
So you've opted to go for the kill option. You have decided to kill.
Me? I've decided to take what action stops the threat with whatever tools I have to hand. A rugby tackle will be as effective as many other options, though if the target isn't stunned I may be the next victim. If the target is killed or seriously injured then it is likely I will spend at least a few nights in jail on a manslaughter charge. With your mindset, even if they only got a bruise I could be facing a "wounding with intent to kill" charge - but that's NOT why I have decided I will stop them being a threat with minimal force necessary.
It could be I club them across the back of the neck/shoulders with a large rock, nearby child, or something else solid enough to stun them and hopefully not kill them or hurt them seriously. Baseball bat or golf club to the legs can take someone down quite effectively. And a hockey stick between a mans legs from behind with a quick upwards and backwards jerk will mean he ain't thinking about his intended victims for a while, plenty of time for them to get away and for you to restrain the perp. I keep cable ties in the car in case bits want to leave it (not that it has happened or is likely to) or need to fix cabling somewhere with a quick-n-dirty method. They'll make sure they're not going anywhere without a police escort.
If I was to kill someone during defence, it would be by accident. I tackle them and their head hits the kerb in a bad way, I move on them and in a panic they cut themselves and sever an artery, so on and so forth. I focus not on "how do I kill someone while defending another" (and, it seems for some, not on "and how can I find a situation where I have an excuse to justify this") but "how can I quickly stop someone so they're no longer a threat without permanently harming them?", After all, the guy trying to stop his wife and her sister taking his kid from him might actually turn out to be a guy trying to stop his kid from being kidnapped by two strange women; the guy on a rampage in a local restaurant may just have learned that his son was raped by a teacher - the same teacher the guy had raised concerns about 6 months ago.
People break, have an episode, maybe hurt someone, then heal. Do you want to add to the hurt by adding more victims, or do you want to stop the hurt with the minimum death&maiming possible?
Value life, protect life - you can still be effective at stopping violence without killing. But first you have to decide that you will not kill.
Oh, that sniper on the rooftop of the building across the road where he's got his gun trained on a number of people and your only option is to shoot him? Remember the gun safety law of "identify your target" - that also means making sure he is a sniper and is intent on evil purposes.
The same as if I had to mow someone down with my car to save other people I would
Most likely you will hit your target and, having totally misjudged your speed, braking distance, and the damage a car does to flesh, continue on and also kill the people you were trying to protect. Almost as likely, your target will dodge out of the way and in that moment of confusion, your foot still on the gas, you'll hit and kill more people than the perp was interested in harming.