But there is another form of homicide which is excusable in self-defence. There are cases where the necessity for self-defence arises in a sudden quarrel in which both parties engage, or on account of the initial provocation given by the person who has to defend himself in the end against an assault endangering life. In these cases, in order to render homicide excusable in self-defence, it has been said that the party must retreat “to the wall” before striking fatally in self-defense. It seems that the first assault ill a sudden affray, all malice apart, will make no difference, if either party quit the combat and retreat, before a mortal wound be given. [Foster, p. 277]. So if A, upon a sudden quarrel, assaults B first, and upon B’s returning the assault, A really and bona fide flies, and being driven to the wall, turns again upon B and kills him, this will be se defendendo.
[1 Hale, 482, Russell, p. 771, 8th Edn.].
It has been observed that homicide upon chance medley borders very nearly upon manslaughter, and that in practice the boundaries in some instances are scarcely perceptible. In both cases it is presumed that the passions have been kindled on both sides, and blows have passed between the parties; but in manslaughter, it is either presumed that the combat has continued on both sides till the mortal stroke was given, or that the party giving such stroke was not at that time in imminent danger of death. [Foster, 276-277].
The distinction between manslaughter and excusable homicide or chance medley, is thus stated by Sir William Blackstone: when both parties are actually combating at the time the mortal stroke is given, the slayer is guilty of manslaughter; but if the slayer has not begun to fight, or, having begun, endeavours to decline any further struggle and afterwards being closely pressed by his antagonist, kills him to avoid his own destruction, this is homicide, excusable by self-defence.
[4 Black Comm 184]