Could you educate us on that "ancient common law"? My understanding of historical conventions doesn't indicate much of a "duty to retreat".
http://www.sagepub.com/lippmanccl2e/study/supplements/Ohio/OH_08r.pdf
The basic idea is that in medieval England violence was common, far more common than it is today even in the worst cities.
Brawls broke out commonly. If somebody died in one of these brawls, for the killer to claim self-defense to justify or excuse the killing, he had to show that he was not a voluntary participant in the fight. That he attempted to withdraw but was unable to do so.
This was part of the common-sense, at least to me, notion that if two men get into a fight and one gets killed, the survivor cannot automatically claim self-defense.
It seems to me the major flaw in the "stand your ground" law (of which I approve in principle) is that it assumes all confrontations that end in violence have one participant who is the criminal aggressor and another who is an innocent victim of aggression.
Anybody who has ever seen a schoolyard or bar fight knows this is often not the case. Many fights have no obvious initiator or victim. Both participants no doubt feel justified in their own minds that they are defending themselves, but this is not always so obvious to an objective observer.
Duels were common in 18th and early 19th century America. Each participant was attempting to kill the other. Should this have allowed the survivor to claim self-defense, since he indeed killed a man who was trying to kill him? Legislators of the time didn't think so. They outlawed dueling.
Certainly many violent encounters are of the aggressor/victim type, but by no means all. And in those less clear-cut situations I think it is a bad idea to allow the survivor to automatically claim self-defense.
For instance, in the Zimmerman case, it seems at least possible that had the gun killed Zimmerman, Martin would have had a case for self-defense.