Supposing persons A and B are having an argument. At some point A pulls out a gun, points it at B, and says clearly, unambiguously, that he is going to kill B. But rather than immediately following through A puts the gun away and stops threatening B (but doesn’t do or say anything else to make B think he did not mean his threats). Person B (who is not armed) takes advantage of the fact A has put his gun away to attack and kill A.
Lets say take this place in a large flat area with no cover, and A is a known to be a crack shot (so if B has decided instead to run away, and A had decided to follow through with his threats he would have no problem shooting him in the back).
The whole thing is witnessed by unimpeachable witnesses. Given the jurisdiction of your choice, will B get off by arguing self defense?