In the United States, you can use deadly force in the defense of another person when it is necessary to prevent death or serious bodily harm. This is known as the “defense of others” doctrine. Generally speaking, you must have a reasonable belief that the other person is in imminent danger and that your use of deadly force is necessary to protect them from death or serious injury. Additionally, the amount of force used must be proportional to the threat posed by the aggressor—you cannot use more force than is necessary to stop the threat.
It’s important to note that different states may have their own laws regarding when deadly force can be used in self-defense or defense of another person. It’s critical to understand and abide by these laws before taking any action. If you’re unsure about what constitutes legal self-defense or defense of another person in your state, it’s best to consult with an attorney who specializes in criminal law for guidance.