You're entitled to that opinion but I personally find it morally reprehensible. We discussed a very similar situation in my CCW class, do you shoot someone in the back after they stole your TV and are already halfway across your front yard? Half of the people in the class agreed with your thinking, the other half agreed with mine. "Well he's still on my property so legally I can just smoke him".

A lot of people think they're totally justified in shooting an unarmed intruder in their home as well, which legally in most states is accurate. However if I could get them to leave by pointing my gun at them and asking them nicely (or not so nicely), I'd much prefer to do that. I don't understand how anyone could be so eager to take another person's life unless it's absolutely necessary (and if they're running away with their back to you it's not necessary).

If someone is running away from you they cannot possibly be a threat. If they turn around to re-engage with you? Okay, now they're once again a threat and you can shoot them. If they run away, get a gun, and come back? They're now, once again, a threat and you should respond according. But in the act of running away they're no longer a threat.