Here are my thoughts on the situation. Take it how you wish, but I think a perp that is still on your property should still be shot in the back. Here's why:
1) He just robbed you in your house and/or place of business.
2) If you are contemplating armed action, the perp is most likely armed
3) If you just got robbed, he is running out the door still armed.
4) If he's still armed, there is a potential that your friends/family/customers/suppliers are outside the door and could encounter an armed robber
5) You could be just the first of many on a list of targets to get robbed
That being said, you are obviously in imminent fear for your life. Once the guy turns his back to leave, he is no longer an imminent threat to you, but then becomes an imminent threat to others.
In most C.D. states, imminent threat to the lives of others is a defensable use of deadly force. In our TN Carry class, we were taught that a shot in the back is still justifiable if the perp is within the confines of your building because you don't know his next intentions. Is he running out the door or to your child's bedroom? If he's running out the door, is there someone outside that he can attack?
I say, "Pool ze treegor oonteeel eet goes 'CLEECK'"