A punches B
B beats up A
A ceases fighting
B does not stop
by not stopping B has become the aggressor
A can now attempt a self-defense justification on lethal force to stop B's assault.
-This does not absolve A of the initial criminal assault however, there is simply now 2 criminal assaults that have occurred; one by A and one by B.
It is a legal parallel to the person who defends his home or family or business from an intruder. If you break into my house and I let you have it with both barrels of Ole Betsy, then I am legally justified. However, if I reload and then shoot you again while you are down, I have committed murder (that is why having a large capacity magazine is great, no reload window).
There have been some criminal cases of this nature, and many civil ones.