I agree wholeheartedly with castle doctrine. But I do think that it rightly ends at the castle. The possibility of a reprisal in the future doesn't give him the right to be an executione of a fleeing or wounded former attacker. I don't think invading a home should merit a death sentence unless you're still in the home, or at the very least still on the person's property.
I'm torn on the matter. I've never served, but I think using a military distinction would be appropriate. Is a fleeing combatant still a legal target in battle?
In "Reports of International Arbitral Awards, Volume 26" (see Eritrea/Ethiopea D.1.39) it is suggested that fleeing militia are still enemy combatants and legitimate targets. I would be hard pressed to hold an untrained civilian to a higher standard than professional soldiers against someone who means him harm individually. I would suggest that under this standard he has the right to engage until combat ends.