Like all legal things in self defense related cases, it's tricky. It's one thing if your assailant was using something like a knife or their fists to hurt someone and then opts to flee. At that point, they no longer have the capacity to harm others as long as everyone keeps their distance, so unless they're perceived as being on the move to hurt someone else (which you would need to prove), you're right in that that would be a bad shoot. However, that sort of proximity phenomenon is off the table with a gun. You can very easily fire a weapon while retreating, and someone with a gun on the move can still pose a threat almost instantly. The assailant firing blindly without even turning is a possibility, and all they have to do is get lucky. All this to say, it's not super clear cut when a gun is perceived to be involved, though his perception on the presence of a firearm is a different discussion and one that will certainly come up alongside this.