We have a sticky on that. Look for Castle Doctrine.Posted by silicosys4: I'm pretty sure its not legal to shoot someone in self defense for the act of opening your front door in any state.
In many states, a defender's having had a basis for a reasonable belief that someone had entered, or in some states, was attempting to enter, one's domicile unlawfully creates a presumption that deadly force was justified. In other states the entry or attempt must be made "with force"; in some places that would entail turning the knob; in others the entry must be "tumultuous". In some places the law encompasses the automobile. In all cases, the structure or the auto must be occupied.
However, shooting someone is not something that one ever, ever wants to do unless it is absolutely unavoidable. By the way, that applies to any use of deadly force, whether administered with a gun, blade, bat, crowbar, nine iron, or Cross pen.
One other thing: all such presumptions can be rebutted. If the state were to have sufficient evidence to prove that the defender had some basis for a reasonable belief that the person shot had not represented a threat, things could get ugly.
Uglier, that is. If you shoot someone, you can expect to end up in all kinds of unpleasantness, from neighbors, co-workers, children's friends and teachers, prospective employers, and the demons in your own mind, every night, perhaps forever. And that's even if you had to do it.
A good read on that is In the Gravest Extreme, by Massad Ayoob.