Bob threatens John with a gun. Alice, who is also carrying a gun (legally), draws her gun and aims at Bob, intending to shoot him in defense of John, who is unarmed. John says to her, "Don't shoot him!" Alice shoots Bob anyway. Is this legal?
Assume that it would have been unquestionably legal had John consented or remained silent.
Alice's defence will be that she had an honest belief, given the circumstances, that force was necessary and the force she used was reasonable in defence of John (and possibly Alice).
John's consent is irrelevant unless it had some bearing on that.
Why did John oppose the use of force?
Did John tell Alice not to shoot because he would rather die than cause a death? Irrelevant.
Did John tell Alice not to shoot because he believed Bob was not a real threat due to circumstances X, Y and/or Z that he wanted Alice to heed? Relevant.
See
If Alice believes on reasonable grounds that there is a threat of force against John, then the defence called "defence of another" comes into play. It is codified at s. 34 of the Criminal Code:
34 (1) A person is not guilty of an offence if
(a) they believe on reasonable grounds that force is being used against them or another person or that a threat of force is being made against them or another person;
(b) the act that constitutes the offence is committed for the purpose of defending or protecting themselves or the other person from that use or threat of force; and
(c) the act committed is reasonable in the circumstances.
"Unless the accused subjectively believed that force or a threat thereof was being used against their person or that of another, the defence is unavailable" (R. v. Khill, 2021 SCC 37, para. 52). Such belief must also be based on reasonable grounds. They must also act with the subjective purpose of protecting themselves or the other person from that use or threat of force. There is also the final balance of reasonableness of the response, but I don't take you to be putting that into question.
As long as these are satisfied, then the defence is made out. And you haven't given enough information to know whether these elements are present.
The scenario you describe only presents evidential uncertainty to the trier of fact. John's shout to "Don't shoot him" would just be part of the evidence relevant to establishing the reasonableness of Alice's belief that John was under the threat of force.
germany has §32(2) StGB
Notwehr ist die Verteidigung, die erforderlich ist, um einen gegenwärtigen rechtswidrigen Angriff von sich oder einem anderen abzuwenden.
Self-defense is the defense which is necessary to defeat a present, unlawful attack against oneself or another. (my translation, emphasis by me)
The question here becomes if John saying "don't shoot him" should make it clear to Alice that Bob is not making an unlawful attack. "Don't shoot him" is not the same as "I consent to being attacked," and John cannot lawfully consent to being killed that way, anyway. It is possible to consent to being injured (many medical procedures would be assault if there is no consent), but a gun is not a suitable instrument for that.
Any specific case would almost certainly be argued in court. You specified that without John's statement, the self defense would be found legal, and these circumstances are not altered by a shout in a confusing situation.
