In order to use lethal force, the person using that force must be in imminent danger. The Blacks Law Dictionary (https://thelawdictionary.org/imminent-danger/) defines imminent danger as:
In relation to homicide in self-defense, this term means immediate danger, such as must be Instantly met, such as cannot be guarded against It calling for the assistance of others or the protection of the law. U. S. v. Outerbridge,27 Fed. Cas. 390; State v. West, 45 La. Ann. 14, 12 South. 7; State v. Smith, 43 Or. 109, 71 Pac. 973. Or, as otherwise defined, such an appearance of threatened and impending injury as would put a reasonable and prudent man to his instant defense.State v. Fontenot, 50 La. Ann. 537, 23 South. 034. 09 Am. St. Rep. 455; Shorter v.People, 2 N. Y. 201, 51 Am. Dec. 280.
I see the words; “immediate danger,” “instantly met,” and “threatened and impending injury.” I also see “cannot be guarded against it by calling for the assistance of others or the protection of the law.”
Massad Ayoob teaches about the AOJ triad in his MAG-40 training and Andrew F. Branca discusses it in his book, The Law of Self Defense. This triad consists of ability, opportunity, and jeopardy.
The elements of the triad are considered when determining if self-defense actions meet the immediacy test.
The first element is ability. Does the threat have the ability to cause you harm? Of the three legs of the traid, this one is least likely to negate the self-defense argument. Having the ability to harm someone else is just about universal. The degree of harm can certainly be a point of discusson, but that is a different leg of the triad.
The next element is opportunity. Does the threat have the opportunity to cause you harm? Someone seeing you walking on the ground floor of a mall and begins threatening you with a knife from the second or third floor doesn’t have the opportunity to carry out the treat immediately. That is a very simplistic example. Branca’s The Law of Self Defense gives several examples to include case law from various states in how opportunity is adjudicated.
The third element is jeopardy. Does the threat, with ability and opportunity, show intent to cause harm to you or another innocent party? That places the innocent party in jeopardy.
An example in Branca’s book is a security guard in a bank. He has the ability and opportunity to do harm but, unless you are a threat yourself, has no intent to cause harm. Thus, no jeopardy.
Once these three elements exist, deadly force can be used. That is, as long as it is proportional to the threat.