For Texas, the way I see the idea of "threat assessment" and using a gun is divided into two categories. First is protecting my life or that of a loved one or friend. Second, we have certain laws that allow us to protect our property after dark such that lethal force may be used if we feel that is the only way to protect the property. At this point, please understand that I am not saying I want to shoot somebody for stealing sprinkler heads out of my lawn, but the provision is there that allows use of lethal force. A few months back, a kid was shot in the back by a man who was raising prized fighting cocks. Two boys decided to steal several of them and for those of you who know about chicken coups, it is hard to raid one quietly. One boy was killed, the other got away. As I recall, they were highschool student. There was a lot of outrage over the needless death and that the owner of the cocks should be locked away for murder. I don't believe he even went to jail (which is surprising) because as it turns out, he was acting totally and completely within his rights to protect his property. For the kid who lived and the parents of both kids, they learned a valuable lesson that when you engage in a crime against a person or belongings at night, you may very well be truly risking your life.
Like the disparity of force concept, several people interviewed talked about how the cock owner shot the boy in the back and how terrible it was. I have yet to find any US laws that make it illegal to shoot somebody in the back. The horrors and chicken-attitude (sorry for the pun) of shooting in the back are gravely over romanticised with pop western culture. The issue does not come down to where you shot the person, only whether or not you were justified in using lethal force - in the criminal courts anyway. For civil courts, all bets are off.
You know, if some idiot threatens you or is making away with your property (in Texas at night anyway), wife or child and you shoot that person in the back, I personally don't see the error as yours because you were only taken full advantage of a hugely stupid tactical mistake on the part of the bad guy. Just because his back is turned does not mean he is off limits. Let's face it, a lot of animals we hunt end up shot from behind. Nobody thinks twice about that.
I personally don't see a lot of use for the law for my particular situation as I have insurance and so if somebody steals my sprinkler heads, that is an inconvenience I will have to deal with.
I noticed 'disparity of force' was mentioned above. From what I can find out, in some states or jurisdictions, disparity of force is a true legal term that indicates certain parameters. A 100 lb lady can shoot a 200 lb man who is roughing her up a bit because of the disparity of force and that she may not be able to stop the assault any other way. In other places, disparity of force is not a legal issue, but is creatively used in courts anyway.
What is life threatening or grave bodily harm? The first CHL shooting incident in Texas took place one month after it became legal to carry. The shooter shot an unarmed man who had pummelled him with his fists, broke off the assault, and returned apparently to administer additional punishment. The shooter shot the pugilist once in the chest and he died sometime later. You can read more about it here:
http://ptb.org/txchl/defense/022196.html
The shooter was permanently disabled in one eye after the assault. At the time, local news stations blew the event out of proportion noting that the killing was over a slight traffic altercation. That was simply not true. The guy was shot over doing grave bodily harm to the other guy who showed amazing restraint in not shooting him sooner. And this brings up a good point. Somebody pulls a knife on you and says, "Give me your money." You have all of $9 on you. Should you give it to the guy? Maybe so, maybe not. There is nothing wrong with giving up the money but the fact of the matter is if you are threatened with a weapon under the auspices of a robbery, you are still in an extremely life threatening situation. Say you shot the would be robber and it comes out that you shot the guy over $9. Is that true? No, of course not. You shot him because he threatened your life with a weapon. Robbery became immaterial at the moment he produced that weapon and threatened your life. It wasn't about $9, it was about defending your life. The part about the money was only how you got introduced into the life threatening situation.
I am willing to let things go, but I am not willing to let the lives of me or my family go. If somebody burgles my car at night and it is gone the next morning, then I take the bus and wait for the insurance check. Nobody was threatened. If somebody tries to carjack me, then my life is threatened and I will work any way I can to be victorious. Some things matter, some don't.