Rules about self-defense in the US vary from state to state, but generally a person can raise a defense that they legally used deadly self-defense if they had a reasonable fear of death or grievous bodily harm, with some exceptions (exactly how these are worded changes from state to state):
- A person often cannot claim self defense if they are already committing a forcible felony (but it depends on the circumstances)
- A person cannot claim self defense if they are the aggressor in a fight or took aggressive actions toward another person who then attacked them.
So, with that in mind, let's assume that Person A has a valid deadly self-defense claim against Person B. When considering self defense, the question is whether a reasonable person in the defendant's position would have feared death or grievous bodily harm. So, the exact sequence of events matters. Consider the following four scenarios:
C watches B attack A and then A shoots B, C pulls out his gun and points it at A before A turns to him and points his gun in response
C watches B attack A and then A shoots B, A then points his gun at C and C pulls out his gun in response
C doesn't see B attack A and only sees A shoot B, C pulls out his gun and they stand off
C doesn't see B attack A and only sees A shoot B, A then points his gun at C and C pulls his in response
In some of these situations, A may be considered the aggressor against C even though he had a valid self-defense claim against B. In others, a jury may find that a reasonable person in C's position would not expect A to shoot them just because they shot an attacker, and therefore C is the aggressor by drawing on A. This all assumes a hypothetical where these people exist in a void, things like their relationship, the situation around the three subjects, and any other relevant facts could be introduced and considered in the analysis. In theory, a situation could exist where both sides have a valid self-defense claim against each other, the third example possibly being that situation.