This is more a psychology debate that has to do with the fact that the goal of combat isn't necessarily about killing. Roman Gadiator matches were rarely to the death since owners didn't want to lose their investments (and probably one of the reasons their armor was designed to avoid potentially crippling limb injuries). In duels if your opponent has clearly been defeated or is running off then you've defended your honor, you've proven you were the better man so there generally isn't much reason to finish your opponent off.
Personally, I'm not a big fan of the "most casualties are people being stabbed in the back as they flee" theory. A person in flight is still dangerous, from the perspective of a warrior who is concerned about staying alive do you hold your ground and accept victory or do you give chase and risk some lucky strike ending it all?