Actually, it does. You cannot just go around shooting people, even if they threaten you. I have people trespassing in my yard and depositing dog crap on my lawn all the time, and extremely pompous about their flouting the law to do so -- one %^&* even pushed me to the ground, and I'm a woman -- but even though it might stop the abuse, I can't just shoot them.
That depends on where you live. Once the person moves beyond assault to battery, it’s a different story. The Zimmerman jury got that one right. There are places in this country that doing less is justification for killing.