[Rick Lee]

The threshold in most of the US is "if a reasonable person would believe that death or grave bodily harm is imminent." If in your own home or usually your car, you get the benefit of the doubt that you were in fear for your life, as long as you don't shoot a fleeing perp in the back. After all, anyone who breaks into an occupied home or vehicle should be assumed to be ready to kill you because they have to know you can legally kill them. Personally, I would not want to just wound or injure someone who was trying to kill me. In our ridiculous, litigious society, letting the cops hear the other guy's story can mean financial ruin for you, no matter how justified you were. It's best to make sure the cops only hear your side of the story.