Bearing Arms, shall not be infringed.

Deadly force means that force which a reasonable person would consider likely to cause death or serious bodily harm. Its use may be justified only under conditions of extreme necessity, when all lesser means have failed or cannot reasonably be employed. Armed Citizens Stop Criminals
https://www.youtube.com/watch?v=mqLaWcqSOTM Armed civilian shoots and kills Oklahoma City restaurant gunman
https://www.youtube.com/watch?v=7Aj0MAwO0Hk Use deadly force to protect people in South Africa. https://www.youtube.com/watch?v=UhQlnxvqkRU