Which part?
As for defending your family/property I know the laws vary somewhat state to state. In Nevada they have a so-called "Defend Your Castle" law that states if anyone enters your house, or ****s with your property (including family) you are fully within your rights to shoot them dead.
Our house was broken into last year and it kind of freaked me out for a few days (since I had moved here ahead of my family I was alone for a month or so after the breakin). I talked to the cop who came to take my report the night of the breakin and asked what he thought I should do, as in barricade the door (I couldn't get it fixed that time of night) or go stay elsewhere or what. He said "hell if it were me I would sit in the dark with the door wide open holding a shotgun hoping they come back". He said the laws were so strongly on the side of the homeowner in Nevada that as long as you, in his words, "can drag him to within a few yards of a window" you could kill them.
As far as the execution part I described, I am pretty sure it would be considered murder in every state, in those circumstances. Might get off due to temp insanity or something, but having the guy tied up with the police on the way, then shooting him in the head is pretty cut and dried illegal.