When may an aggressor use self-defense against their victim?

Some fact patterns are easy to figure out when it comes to who can use self-defense against whom. For example, if Bob starts shoving Sam, then Sam is justified in shoving Bob out of self-defense. But things can become more complicated. For example, if Bob shoves Sam, and then Sam begins to punch Bob in self-defense, at what point does Bob become the victim and have the right to defend himself even though he is the person who originally started the fight?

The law creates two options:

(1) If Bob "effectively withdraws" from the confrontation and lets Sam know that he has withdrawn, then Sam's continued engagement would make Sam the new aggressor, enabling Bob to defend himself. For example, if in the middle of the fight, Bob escapes from Sam, runs 20 feet away, puts his hands up and says, "Sam! I'm Sorry! I'm done. I'm done fighting you. I'm sorry!" and then Sam ran after Bob to continue the fight, Bob has the right to self-defense even though he was the initial aggressor to the initial fight. Sure, Sam has the right to be pissed off that Bob shoved him and started a fight earlier, but the law doesn't give Sam a free pass to continue punching Bob for the rest of his life. Bob withdrawing and letting Sam know he was no longer a threat puts an end to the aggression and any further aggression would be the start of a new fight.

(2) Imagine if Bob and Sam got into a minor shoving fight and Bob was the initial aggressor. After a few mutual shoves, Sam pulls out a gun and puts it to Bob's head, so Bob pulls out a knife and stabs Sam. Yes, Sam was originally the victim, but his decision to suddenly escalate the violence to life-and-death means that Bob is allowed to defend himself (and his life) even though he was the initial aggressor. In other words, the law doesn't expect Bob to let Sam shoot him in the head just because he started a shoving match.